As it turned out, Lukas Foss underestimated the occasion when he was asked to write a new composition for the First American Cello Congress, which opens today at the University of Maryland. The piece is titled "200 Cellos: A Celebration," and it is written for an orchestra made up entirely of cellos, the only instrument with enough range for that kind of assignment. Ordinarily, 200 sounds like an awful lot of cellos. But yesterday, with last-minute applications still coming in, about 400 cellists were ready and willing to participate in the world premiere performance Friday night in the Tawes Theater.

But the title was right, anyway; the stage in Tawes will only hold about 200 cellos. Half the cellists will have to sit in the balcony and watch while the premiere takes place under the baton of Mstislav Rostropovich. Their turn will come in the second half, when they join eight double basses, seven harps and tympani in a performance of Glazunov's entr'acte from "Raimonda" and Davidoff's "Hymn." As spectators and performers alike, they will be participating in a historic event: the formation of the largest orchestra of cellos in the history of music.

One cellist, the most famous of all the big-name performers at the event, will not be playing his cello in that performance. Rostropovich, who is the president of the congress, will be busy conducting the concert. It will not be the first time he has conducted a cello orchestra. Last June, he organized 50 cellists for a concert in the Zurich Tonhalle marking the 75th birthday of Pierre Fournier and he is looking forward to repeating and expanding the experience. When he talks about the cello, his voice takes on the tone of a lover discussing his mistress.

"No other instrument--only the piano--can be compared to the cello," he said on a recent evening in his suite at the Watergate. He was seated at his piano, occasionally playing a pair of notes a few octaves apart: "From here . . . to here"--showing, in turn, the relatively limited ranges of violin, oboe, clarinet, piccolo and double bass. "But the cello . . . from here to here," and his arms stretched out almost to the limits of the keyboard. "Only the cello can give you such a wide range in the same color of sound.

"The ideas in this music we will play on June 4 may not compete with those of Bach and Brahms," he said, "but the beauty inherent in the sound will be such that no one who hears it will ever forget it."

Beautiful sounds and musical history are only two of the things that 400-odd cellists will be making this week on the College Park campus. There are also plans to make a new organization, a sort of national cello forum (nobody is calling it a "lobby") to represent the interests of the thousands who play the instrument and the hundreds of thousands who love its sound. A small, private dinner will be held on Thursday evening bringing together representatives of cello clubs and societies across the United States with people from national service organizations such as the American Symphony Orchestra League, Chamber Music America, the National Endowment for the Arts and the Violin Society of America (despite its name, an organization of those who make all kinds of stringed instruments, including cellos). From this dinner meeting, if all goes according to plan, may emerge an American Cello Council, a coalition of all organizations in the country that have the interests of the cello at heart. "Each of us is looking for his own path," says Rostropovich. "Why shouldn't we all get together and discuss it?"

Those interests include the development of new repertoire and teaching techniques for the instrument, promoting technological progress in such peripheral matters as the bow and the carrying case, advising young cellists on how to make a career with their instrument and presenting a united front in dealing with airlines. At present, cellists have to buy a ticket for their cello when they travel and keep it on a seat next to them. Cargo holds get too cold for the delicate instruments--but cellists feel that some sort of adequate storage area or protective container could be provided at less cost.

Rostropovich, who spends a lot of time in international travel, is particularly eloquent on the subject. "I am fighting the airlines for making us put the cello in the cargo hold or buy a ticket," he says. "If an employe of Pan Am tells me to put my cello in the cargo hold, I tell him, 'Please put my cello in my seat and give her a drink--only a soft drink, no alcohol, because she has a concert tonight--and I will get in the cargo hold.' I know I would die, but my cello must keep its beauty. My cello was made in 1711 by Stradivari and has served humanity for 270 years. It played the 'Archduke' Trio when it was played for the first time and it played Beethoven's Second Cello Sonata with Beethoven at the piano. Napoleon once tried to play this cello and he made a big scratch on it that you can still see. So I would be perfectly willing to switch my body for the cello in the cargo hold."

By the end of the cello congress, such a sacrifice may not be necessary. Advance reports indicate that a new kind of case may be introduced that will protect the cello against the rigors of a cargo hold. Rostropovich thinks that the time has come to apply modern technology also to the form of the cello's bow--to make it more comfortable for the player and to produce a sound suitable for the large concert halls of today. "We used to play for audiences of 450 or 500; now we play to 2,000, and we must adjust our scale to our new audience."

In the size of its audience, as in the number of players who sign up to join a cello orchestra, the instrument seems to be almost a victim of its own success. Most of the problems that will be discussed at the cello congress seem to be problems connected with growth. One such problem is choosing a time and place for the next cello congress--preferably a place whose stage will have room for an orchestra of 500 cellos or more.

"Unless I'm forced on the issue," says George Moquin, executive director of the university's Summer Institute for Creative and Performing Arts, "I'd rather not do it again next year. We'd love to do it here if we could, but the other instruments are already after me--tubas, saxophones and clarinets all want to have the same kind of meeting. We have a World Congress of Tuba and Euphonium Players definitely booked for next summer."

Perhaps Lukas Foss should get busy now on a piece called "10,000 Tubas."