Like most of the 15,000 troops he commands, Salvadoran Defense Minister Jose Guillermo Garcia is fiercely proud that the armed forces here withstood a January guerrilla offensive "without a single cartridge" from the United States.

But although he laughs at the idea that an ever-growing U.S. military presence is needed ultimately to win the guerrilla war, Garcia scoffs equally at the idea that he would turn down the American arms and advisers who have begun to play a growing role here under the Reagan administration.

"Every army in the world asks help of others," Garcia said in an interview, "even if they have everything." Right now, he said, the Salvadoran armed forces need not only to restock their war-sapped arsenals, but also the American expertise to develop what he envisions as a 1,000-man rapid deployment unit to hit the guerrillas where they live -- in remote hilltop and village bunkers.

He does not want "hundreds" of U.S. advisers, Garcia chuckled, "only those who are absolutely necessary. That is not so many."

Perhaps more than any of the tangle of politicians and colonels who have played a deadly serious game of musical chairs here over the last year and a half, Garcia has a habit of getting what he wants. Junta members and Cabinet ministers have resigned or been forced out in wholesale lots, the entire structure of the government has been reformed, but he has retained the same post throughout it all.

Garcia represents not only the high command of the armed forces, but all the ambiguities, strengths and weaknesses of the institution on which the Reagan administration is depending to "draw the line" against international communism.

At 46, he is the old man of a government first brought to power 16 months ago in a coup by the so-called Military Youth. Garcia was once considered for the presidency under the old military dictatorship but was passed over in favor of the more hard-line conservative Gen. Carlos Humberto Romero. Those who staged the October coup insisted that Garcia remain in the government to reassure those old-line officers who had not been removed.

Garcia is a protege of Col. Jamie Abdul Gutierrez, a member of the original junta that replaced Romero following the coup, and Garcia's appointment as defense minister was the only Cabinet post the coup makers demanded of the civilians who joined them in the coalition government.

The civilians objected, but it was a battle that they, and civilians in the government they followed them, were to lose. Today, Gutierrez and Garcia are the only remaining remnants of the original junta.

Garcia is denounced by the left as the single most onerous representative of the old guard allied with the old economic oligarchy who remains in the armed forces. It was the question of Garcia's fate that more than any other single factor caused the resignation of every civilian in the broad-based, leftist oriented junta formed after the bloodless Oct. 15 coup. Many of those who resigned subsequently made common cause with the guerrillas, including Guillermo Ungo, a member of the first junta who is now president of the opposition Democratic Revolutionary Front.

The showdown came in December 1979 when the country's military commanders confronted the civilians in the junta at a meeting in the presidential offices.

Garcia says the civilians in the government "miscalculated" by demanding changes in the armed forces. They were, he said, "crazies."

His description of the incident suggests the nature of power in El Salvador.

"We told them, senores, why? You're not the ones who put us here. The armed forces put you in here. The armed forces have us in here. If the armed forces tell me to leave, I'll leave. But for you to tell us -- well, you've been installed by us! You're being ridiculous."

"They didn't know how to play the political game." said Garcia. "They came in as if this were Cuba here . . . a complete mess."

"They lost the dimension of where the power was," he explained. "And they perhaps overestimated their power, thinking they could destroy the armed forces, which was their great problem. And we thought we had an obligation to defend the armed forces as an institution. Because after all the armed forces sponsored all these changes. We are the ones who implemented them. The left accused us of opposing them," said the colonel, referring to a series of sweeping economic reforms, "but in fact they were the ones opposed."

Now the emphasis has shifted from reforms to ending the insurgency, although both the Salvadoran government and its backers in Washington still stress their support for the reforms proposed by the Military Youth after the coup and carried out by the sucessive juntas.

The Salvadoran military reported that by the time the fighting slowed down in the last few days of January government forces were running low on ammunition and their backup resources were dwindling rapidly. Of their 10 helicopters only two were in any shape to fly. Only three of 11 patrol boats were functioning and only one of those had working radar.

Resupply was necessary and even though not all of what Washington sends them is exactly what they need -- the M16 rifles and ammunition, for instance, are not compatible with their standard issue NATO-round G3 rifles -- the Salvadoran Army is glad to have them.

During the three-year period from 1977 to 1980 that the Carter administration had blocked military aid to El Salvador on human rights grounds, the armed forces purchased most of their weapons from Western Europe, Salvadoran and U.S. sources said.

As of yesterday, according to the U.S. Embassy, 19 of the 42 U.S. advisers here were concerned with training and maintenance for the newly arrived U.S. helicopters and the Salvadoran patrol boats.

There are still at least 2,000 well-armed and well-trained guerrillas in the field by Garcia's estimate. The U.S. Embassy says there may be twice that many. Although the insurgents have not defeated the Salvadoran armed forces, neither have the armed forces defeated the insurgents.

In addition to the 19 training and maintenance specialists, there are already five members of the U.S. Embassy's military group and another seven-man team specializing in administration, logistics and communication at work here, the embassy said. There are also eight members of what is known as an operational planning team with two more on the way and three members of special small unit training teams laying the groundwork for 12 more small unit trainers to come.

"We're not going to clean the guerrillas out overnight," said Garcia. "These people are well armed. They continue to get help. They continue to get ammunition. They continue to get arms. That is why we insist on military aid -- because we need it."

Reports from rural areas indicate that the guerrillas have been forced out of most of the villages they once held, but their ranks remain intact and they are regrouping.

At a news conference today, Garcia said the armed forces still do not have "total control" in the northern and eastern parts of the country. He said government forces have killed about 2,200 guerrillas this year, while only 147 government soldiers have been killed in the fighting.

Unidentified airplanes suspected of transporting arms to the insurgents are still being sighted, according to military sources.

"It is difficult, very difficult to eradicate guerrillas," said Garcia. Even if they can be defeated in the field, terroist activity may continue in the cities "until they [the rebels] run out of dynamite. We can't control that. How? No country in the world can halt that kind of terrorism."

Virtually all parties in this complex fight are now talking about the need for a political solution. In that as much as in the open fighting, Garcia stands as a key figure. He represents not only the high command for the armed forces but all the ambiguities, strengths and weaknesses of the institution on which the Reagan administration is depending to "draw the line" against international communism.