JIMMY CARTER knows exactly how many times he has been on the cover of Newsweek. "When I was a candidate I was on the cover seven times in an 18-month period," said the former president at Newsweek's 50th-anniversary extravaganza at Lincoln Center last night. "They tell me I broke a record. I guess I was a curiosity. Newsweek helped get me there. They have been nice to me."

Halston never made the cover. "But I did have a girl on the cover once, and the story was about me," he said. George McGovern wasn't sure. "At least two times," he said. "But I think it was three . . . Actually I think it was four . . . Make that five."

The word circulating around Lincoln Center was that Henry Kissinger had broken all cover records for a non-head of state with 14 times. But Kissinger was backstage rehearsing for his part in the program and not to be found in the throng at the cocktail hour preceding Newsweek's gala.

Many others, however, could be.

Liza Minnelli walked arm in arm with Halston. Jimmy Carter kissed Mary Tyler Moore hello. Presidential counselor Edwin Meese introduced Phyllis Schlafly around. Attorney General William French Smith stood in deep conversation with Shirley Temple Black. And Katharine Graham, chairman of The Washington Post Co., led First Lady Nancy Reagan by the hand through the crowd. In the words of one guest caught between the dozens of exploding flashes, "Is there anybody who isn't here?"

Asked how she would get through the evening, Katharine Graham answered concisely: "Endurance." When someone congratulated her on 50 years, she responded, "I wish it were me--not the magazine."

This month Newsweek magazine hit the half-century mark and not even the winter's worst slush deterred the nation's top newsmakers from coming to the party.

The guest list included, from the world of politics and government: Mrs. Reagan, former president Carter and his wife Rosalynn, former secretaries of state Kissinger and Alexander Haig, Secretary of Defense Caspar Weinberger, U.N. Ambassador Jeane Kirkpatrick, Austrian Chancellor Bruno Kreisky and Israeli Ambassador Moshe Arens.

From the arts and letters: novelist Bernard Malamud, architect I.M. Pei, impresario Joseph Papp, photographer Richard Avedon, actor Jason Robards, actresses Moore, Sissy Spacek and Raquel Welch, directors Steven Spielberg and Robert Altman, singers Diana Ross and Minnelli, artists Andy Warhol and Willem de Kooning.

From the media: CBS' Walter Cronkite, Time editor-in-chief Henry Grunwald, Washington Post executive editor Benjamin C. Bradlee and CBS commentator Bill Moyers. From business: executive Laurance Rockefeller, oil magnate Armand Hammer, CBS Inc. chairman William Paley. From sports: former quarterback Joe Namath, tennis champion Martina Navratilova, sports-car racer Mario Andretti. And assorted others, among them designers Halston, Calvin Klein and Bill Blass, evangelist Billy Graham and journalist Tom Wolfe. And nearly 2,000 more.

In a city where politics is a curiosity rather than an obsession, the Washington presence at times overwhelmed the famed faces of New York's arts, literary and financial establishment.

"Washington is bringing some class to New York," decreed Art Buchwald. "We won the Super Bowl and now it's time to show New Yorkers how we really live."

"I feel like a provincial here," said OAS Secretary General Alejandro Orfila, one of a number of diplomats who came up from Washington, including the ambassadors of Japan, Egypt and Sweden.

Planning for the event began a year ago; the celebrations include seven anniversary parties and a special issue of the magazine Feb. 21. "We sold double the amount of advertising in our special anniversary issue," said George Simpson, Newsweek's director of communications. "We generated an awful lot of revenues that we wouldn't have had in a normal year." Limousines began discharging the celebrity-studded black-tie crowd around 7 p.m. Cocktails were served, and dinners followed, at two separate locations: the marble and glass Avery Fisher and New York State Theater halls. Katharine Graham moved between both halls. She welcomed former president Carter and his wife at the Avery Fisher Hall reception and Mrs. Reagan at the State Theater Hall reception, and presented them with special leather-bound editions of a history of Newsweek, "A Draft of History." At the head table with Mrs. Graham in the State Theater were Carter and Mrs. Reagan; Mrs. Carter dined at the Avery Fisher dinner. They all dined by candlelight on rack of lamb, haricots verts, rice with grapes and apple brown betty.

Chandeliers twinkled and enlarged Newsweek covers hung on the walls throughout Lincoln Center. The press corps was chaotic, shoving microphones, notebooks and bright lights into the familiar faces.

It could have been a presidential candidate's heaven. But, of course, no one was declaring.

"I'm supporting Alan Cranston," said Jerry Brown. "I'm on sabbatical--involuntary sabbatical--but I'm enjoying it."

"I'm not leaning in any direction," said McGovern about all the Democratic hopefuls. "But I feel there's a good field out there to choose from."

"I'm not actually going to get involved in the primary season," said Jimmy Carter. "I think the Democrats should fight it out themselves."

Even Liza Minnelli, whose media exposure is carefully calculated, seemed eager to talk. She said she was on her way to California to work with Blake Edwards on two films, and was putting together a retrospective exhibit of the films made by her father, director Vincente Minnelli.

Shirley Temple Black said she was also celebrating last night. This year is the 50th anniversary of the year she went to work for 20th Century-Fox.

Meanwhile, several blocks away, at the New York Sheraton, 900 Newsweek employes attended a buffet dinner. At 9 p.m. they boarded buses for Lincoln Center where the elaborate birthday show included a performance by the New York City Ballet.

The program also included humorous and historical recollections about the magazine that started out as "News-Week" in 1933 when Thomas Martyn, a former foreign editor of Time magazine, was determined to give his former employer some competition. The first issue was published Feb. 17 by an editorial staff of 22 and had a circulation of 50,000.

Former Newsweek editor-in-chief Osborn Elliott, who was involved in the original sale of Newsweek to the late Philip Graham, then president of The Washington Post Co., introduced the program participants with Katharine Graham. They included Kissinger; civil rights leader Vernon Jordan; columnist George Will; actor James Earl Jones; actresses Lauren Bacall and Jessica Tandy; Brooke Astor, whose husband once owned the magazine; and others. The show was produced by Lorne Michaels, former producer of the "Saturday Night Live" TV show. He also is videotaping portions to be shown at subsequent anniversary celebrations around the country.

The program opened with a slide show of 50 years of history as reflected in Newsweek covers. Then Mrs. Graham welcomed Nancy Reagan and the Carters. "Well, I think that takes care of the A list," she joked, referring to reports that the receptions had been split up according to guests' order of importance. Earlier, she had told the press those reports were "pure fiction."

Buchwald told the audience, "You are probably wondering what I am doing here tonight on a program celebrating the 50th anniversary of Newsweek magazine. I am here this evening only for one reason. Katharine Graham said I had to be here.

"When Mrs. Graham speaks, even E.F. Hutton listens."

Kissinger said he had been given the task of discussing 50 years of foreign policy in eight minutes. "It's an impossible task for anyone," he said, ". . . but how can I refuse someone who put me on the cover in a Superman suit?"

Art Buchwald told the crowd his version of the sale of Newsweek to Philip Graham by the Vincent Astor Foundation in 1961:

"History tells us that in 1961 a Periscope item appeared in Newsweek which said, 'One of America's leading foundations is looking for a buyer for one of the leading newsweekly magazines in the country.'

"The item caught the eye of Ben Bradlee, then working in Newsweek's Washington bureau. He immediately called up Phil Graham and told him, 'U.S. News and World Report is for sale.'

"Graham said, 'I'll take it.'

"Bradlee called back and said, 'I made a mistake. Newsweek is for sale.'

"Graham said, 'Okay, I'll buy that. If you've seen one newsweekly you've seen them all.'

"And that's how The Washington Post acquired Newsweek," maintained Buchwald. "Had Rupert Murdoch bought the magazine we might be all celebrating Newsweek's 50th anniversary at Plato's Retreat this evening."

History has a slightly different version of how Philip Graham came to acquire the magazine. According to Elliott, after Vincent Astor's death the Astor Foundation didn't want "this cockamamie magazine" in its portfolio. Elliott and Bradlee mulled over how to persuade Graham to buy it. The selling price was $15 million. When Graham agreed, he went to New York to make a $2 million down payment. In the end, with other assets factored in, the purchase cost Graham $9 million, according to Newsweek.

"All he had was a personal check," recalled Elliott. "He crossed out his own name printed on the check and wrote in The Washington Post Company. He said later he had never written a check for that amount of money and didn't know if he should put zero zero cents after the two million. He finally decided to go 'squiggle, squiggle, squiggle.' "