ARE THE Communist leaders of Cambodia waging genocide there? It's widely assumed that they are. But no one knows.

How many Cambodians have been killed since the end of the war nearly three years ago? Tens of thousands? Hundreds of thousands? A million? Two million? All these figures are used. But no one knows.

What's the population of Cambodia? Five million? Eight million? What's the strength of the army? Forty thousand? One hundred thousand? All these figures are used, too. But no one knows.

If nobody really knows much about what is happening in Cambodia, why do most Americans assume that the Cambodian Communists run the most brutal regime since the Nazis? Is the answer, as the Cambodians and their tiny handful of foreign friends allege, that Western governments and news media are guilty of "distortions and wild fabrications?"

Even those few friends of Cambodian acknowledge that they have little in the way of hard facts. Their arguments with popular opinion are based chieftly on what they consider skewed interpretations of the scanty information available. They maintain, in essence, that a number of U.S. government officials have a vested interest in putting the worst possible face on everything happening in Cambodia.

As to press coverage, most journalists who've ever attempted to "cover" Cambodian probably would concede that one guess is as good as another. With so few facts available, it's nearly impossible to prove or disprove anything. Nevertheless, a number of journals, including The Washington Post, Newsweek, Time and Paris Match, have published several photographs purporting to show atrocities in Cambodian.

Several U.S. and other experts believe that these pictures were posed in Thailand. "They're fakes," commented a State Department officer who has followed Cambodian affairs closely since before the end of the war.

Another piece of journalism claimed by supporters of the Phnom Penh regime to be a fraud has been widely used as a basis for the future of 1 million executed. This was an interview supposedly given by head of state Khieu Samphan to an obscure Italian Catholic journal, Famiglia Cristiana, in September, 1976, and subsequently referred to in The New York Times Magazine. Francois Ponchaud, a French Catholic priest who is a bitter opponent of the Cambodian Communists, wrote last August that "I know for certain that the Italian journalist writing in that journal never interviewed Khieu Samphan."

Probably the one published account which most arouses defenders of the Cambodian regime is "Murder of a Gentle Land," a book by Anthony Paul and John Barron. The book received wide circulation through condensation in The Reader's Digest, its publisher, and has given popular credence to the genocide thesis. Its critics complain that it is based almost entirely on refugee accounts and thus is one-sided.

This is not to suggest that whatever is written or said about Cambodia, however horrendous, is not true. What is does mean is that reports about Cambodia should be treated vith skepticism. Interviewing Refugees

ODDLY, THOSE FEW Western governments which have diplomatic relations with Cambodia generally refuse to accept the genocide allegation. "We'd need a lot more evidence before we'd be ready to believe such a serious charge," said an ambassador from a Scandinavian country. Representatives of his government have visisted Phnom Penh several times since the war ended.

No U.S. diplomat has been in Cambodia since April 12, 1975, the day Ambassador John Gunther Dean evacuated the embassy. Just one member of the U.S. embassy staff in Thailand is assigned to monitoring Cambodian affairs.

Most information gathered by this official and by journalists in Southeast Asia comes from interviewing Cambodian refugees who have fled to Thailand. Almost all of these refugees come from the northwestern part of Cambodia, an area which was never well controlled by the Communists and where reprisals by long-embittered guerrillas were fierce in the months immediately following the Communist victory.

From this bare-bones intelligence-gathering, nationwide projections have been drawn. It is these projections that have led to the conclusion that Cambodian leaders are genocidal monsters and that the torment of the once-gentle land has no parallel in modern history.

Why has this been widely accepted? First, while figures may be subject to doubt, what's the difference between whether tens of thousands or a million people have been killed? Any government that orders widespread reprisals against its own people obviously is monstrous. Moreover, a government that does not want to hide some horrible secret would not seal itself off from the rest of the world. If they're not soaking the ground with the blood of their countrymen, why not let independent observers in to look around?

Both these points have acceptable moral bases. Yet they sidestep key issues. For one thing, how many wars have there been, anywhere in the world, which were not followed by reprisals? For another, the Cambodian Communists have never left any doubt about their complusion to free the new society from what they consider foreign contamination. While this certainly may be judged extreme xenophobia, it does not prove that genocide is being carried out behind the bamboo curtain. "What Can I Know?"

NOAM CHOMSKY, the MT linguist who has been one of the most outspoken opponents of U.S. involvement in Indochina, agrees that there have been reprisals and brutality in Cambodia. But he said in a telephone interview the other day, "I do not believe that it is the monolith we're constantly told it is."

Could he prove this? "No," Chomsky replied. "I've followed every scrap of information - every article, every book, the photos, the sketches - and I don't know anything with certainly. What can I know?"

Chomsky said a Cambodian-speaking colleague of his recently toured refugee camps in Thailand and interviewed large numbers of inmates. "He gives some credence of horror stories," Chomsky said, "but he said there were many conflicting stories, often from people who came from the same area."

Furthermore, Chomsky added, his colleague said the refugees confirmed that camp directors normally chose individuals to meet Western diplomats and journalists who did not speak Cambodian "and singled out only whose with horror stores to tell."

In response to criticism about the failure of himself and other war opponents to speak out against the alleged atrocities in Cambodia, Chomsky said the burden was not on them. "My opposition, which was extremely strong, had nothing to do with the nature of the regimes which might arise. I was just opposed to U.S. aggression. Now it's the job of those who supported the bombing, the extremely heavy bombing of Cambodia, to concede that this is what provoked the Communists."

Chomsky charged that former Secretary of State Henry Kissinger deliberately ordered the unprecedentedly heavy bombing, as late as 1973, to "embitter the enemy."

"Kissinger knew in 1973 that the United States couldn't win in Cambodia," he said. "But he was setting the stage for the 'bloodbath' thesis, which is now being given credence. The U.S. record just has to be sanitized."

Gareth Porter, another of Cambodia's few friends in this country, charged that a number of diplomats in the U.S. embassy in Thailand were interested in saving their own reputations. "These people had predicted that millions of people would starve to death once the United States pulled out of Cambodia," he said."When the regime clearly averted mass starvation, these people would have lost face. So they created the genocide claim."

Porter, 35, was a co-director of the Washington-based Indochina Resource Center, which opposed U.S. involvement in Indochina,. He also was a consultant to the House select committee on U.S. servicemen missing in action in Indochina.

Contrary to most views in the United States, Porter insists that the forced evacuation of Phnom Penh and other Cambodain cities immediately after the war's end was well-advised, though "heavy-handed." "The fact is that the evacuation and the regime's concentration on rice production have averted mass starvation," he said. "If you look at the three Indochinese countries today, you'll find that Cambodian undoubtedly is in the best food position." "Very, Very Low Priority"

THIS CLAIM is more or less supported by State Department officials. "Last year's harvest was pretty good, and people are probably eating better," an official said. "There have been reports of some rice exports."

That's about as firm as U.S. officials are willing to be about Cambodia. What about reports that U.S. statellities and electronics-laden ships are monitoring the border struggle between Cambodia and Vietnam? "The capability exists," the official replied. "But we can't discuss details."

What about a report from Bangkok that "six high-ranking Cambodian defectors flew to the U.S. in December for debriefing? "There's nothing to it," another official replied.

Cambodia acknowledges that a group of Communists who had been trained in Vietnam attempted to topple Premier Pol Pot last April and that most of their activity was concentrated in the northwest. Pol Pot emerged victorioua. But is it wise to assume, as has been done, that their plot was more widespread and that another round of executions resulted?

"We don't know that," replied a State Department source. "We don't know how, or if, Vietnam was involved. We think we know who's who is Cambodia now, but we don't know which former leaders, if any, are out."

Why is it that the United States, with its vast intelligence network, should know so little events in Cambodia? The answer seems to be that Cambodia no longer counts for anything in the U.S. scheme of things. At least that's what the officials say. "All of Indochina, as an intelligence target, is of very, very low priority. And Cambodia is so low as to be almost nonexistent," said one official.

Only three years ago, Cambodia was a top priority for this country. U.S. bombers had dropped thousands of tons of explosives on just about every square mile of the country, much of it on densely populated towns and villages. Other U.S. planes airlifted tons of food and medicine to besieged Phnom Penh in the final throes of its agony. By that time half a million Cambodians had died - in battle, in bombing attakcs, from starvation - and Americans had spent neatly $1 billion on the five years of warfare.

So now Cambodia is on the U.S. back burner, receiving only cursory, and often shabby, attention. Since the renegade regime has openly gone to war with Vietnam, which is perceived as relatively stable in the Indochinese context, Cambodia is swiftly losing even its few friends here.

"There's a new hysteria which reveals a worrying dimension of the leaders' instability," said Porter. Although he insisted that charges of genocide were baseless, Porter conceded the the regime was "very, very tough. Harsh. It's a great tragedy."