Even the most cautious observer can lose count at such times, but there were at least 16 full-scale toasts during the 55th birthday party for Mstislav Rostropovich given by the National Symphony Saturday night at the F Street Club.

If that sounds as if things were getting out of hand, the exercise might at least have helped prevent frostbite.

That's because the black-tie dinner for 150 or so guests was so large it had to be served under a tent in the back yard, where almost everyone bundled himself in his heaviest outer wear as temperatures plunged into the 20s. It took about 20 minutes to get people in place as they walked out into the dinner area, tested the temperature and then returned to the cloak room for more padding. There were limits, though, to what could be done. "You can't tell, but I didn't wear any stockings," said one shivering matron. "I'd rather have some hot coffee instead of more wine."

"I have a feeling I am in Roossia," declared the ebullient guest of honor. "I am feeling it more in certain parts of the body than in others, like in the nose."

Rostropovich and his friend and fellow emigre', conductor Maxim Shostakovich, being very Russian, were among the stoic few who adamantly refused to don protection over their tuxedos. But Shostakovich was among the first to seek wraps for others--a heavy fur for Mrs. Rostropovich, soprano Galina Vishnevskaya.

The temperature, and the wine, seemed to intensify the fervor of the tributes. Elaine Silverstein, whose husband, Leonard, is the orchestra's chairman, presided at the dinner and opened with a message from President and Mrs. Reagan proclaiming Rostropovich's "talent" and "love of freedom." There was a similar tribute from the president's national security adviser, William Clark, from whom Rostropovich obtained aid for Shostakovich and his son, Dmitri, when they defected to the West last April 11.

After that, the tributes fell into three categories. First, there was the song-and-dance variety, as in the routine by former arts endowment chairman Livingston Biddle and his wife, Catharina, that began, "Some enchanted evening, you may hear a cello . . ."

Others were more nostalgic, as in Kennedy Center artistic director Marta Istomin's recollection of the first meeting, in 1957 in Paris, between Rostropovich and her late husband, Pablo Casals. They were attending a cello contest and "there was dinner after the first night, and Slava had brought about 24 bottles of vodka. After that, Maestro remarked, 'You know, this young man is very, very smart, but he is a bit loco.' Then later, Slava played a Brahms sonata for Casals, and as he played there were tears in the older man's eyes. And from then on there was great respect for the young man from the old master, and from the young man to the old master."

Chief Justice Warren Burger proposed that he and Rostropovich switch jobs for a day; "It would be good for at least one of the institutions," he asserted. And in his own defense as a musician, Burger said, "Somehow it has been kept a secret from the investigative reporters of the Washington area, but I once conducted the Boulder, Colo., symphony. It was a small group."

Later, Burger picked up a trumpet and played about 10 measures, part of which sounded vaguely like "Taps." As if to fend off criticism, he said, "I'll bet there aren't five people at the party who could get a note out of the trumpet."

Another light moment came when an Eastern Onion maiden appeared as a Star-Spangle-O-Gram, a reference to Rostropovich's repeated mispronunciation of the title of the march, "The Stars and Stripes Forever."

Rostropovich concluded the toasts with an emotional political warning. He is here for the first time since the orchestra's February tour of Europe and expressed distress with the antinuclear movement there. "Americans do not like to suffer. In the U.S. you have such a beautiful life. Your freedom is a gift from the gods. But you must protect it. You don't realize as I do what it is like in the other half of the world. Do not be naive . . .

"Believe me, I love people so much. And I hate war. These demonstrations are not for peace that can last; they are not for peace, but for weakness."

Then he turned to Maxim Shostakovich. The two men embraced, with tears welling in their eyes.