HOW MANY tubas are too many? The answer is about to descend upon us like a swarm of 50-pound brass locusts.

The world got along very well without tubas until about 1830, when the instrument was invented--more or less as a spinoff from the invention of the valve. Before that, when a composer wanted a particular kind of low and vulgar sound, he could always call on the bassoon or a now-obsolete instrument--called the serpent, because that is what it looked like. Mozart and Haydn--even Beethoven--managed to get along quite nicely without the tuba; so did Berlioz, who wrote for a thing called the ophicleide (a sort of bass bugle with keys)--music that is now usurped by the tuba.

Now, here come 500 tubists (that's what you call tuba players) galumphing into the Washington area, planning a mass demonstration on the east steps of the Capitol, making noises in the Library of Congress and organizing all kinds of meetings at the University of Maryland. The city is accustomed to strange gatherings, but this one should set a new standard.

The tuba infiltration begins unofficially and unobtrusively this afternoon with one man--Harvey Phillips, the Paganini of the tuba--giving the first tuba recital in the history of the Library of Congress. For most of the program, which includes his own tuba transcription of the Brahms Horn Trio, he will be abetted by a violin and piano. But for one piece, Edward Salter's "Eight Random Thoughts," the audience will be exposed to his tuba and nothing else.

At the other extreme, at 4:30 p.m. Friday on the Capitol steps, several hundred tubists will participate in the world premiere of "Deep Potomac Bells," by John Harbison, who has been at least until now an admirable composer. Commissioned to commemorate the 350th anniversary of the founding of the state of Maryland, the piece reportedly draws some of its thematic material from the state song, which is known to the rest of the world as a Christmas carol. That's "O Ta nnenbaum," not "Silent Night." There will be no silent nights in Washington while all those tubas are in town.

In between these incursions into Washington, the participants in the International Tuba-Euphonium Conference will be holding competitions, workshops and other sessions in the Adult Education Center on the College Park campus, beginning Tuesday. For the public, there will be concerts in the Tawes Theatre, with the brass allies of the tuba (trumpet, horn and trombone) joining in the festivities, including the Canadian Brass, the Tokyo Bari-Tuba Ensemble, the New York Brass Quintet, the Matteson-Phillips Tubajazz Consort, the U.S. Air Force Band and the U.S. Armed Forces Tuba-Euphonium Ensemble. Tubists are usually a solitary lot, but this will be a week of tuba togetherness.

Why solitary? Symphony orchestras, in their wisdom, limit their rosters to a single tuba, hiring free-lance musicians when they play the music of a power-mad composer who insists on more than one, like Mahler, Bruckner or Stravinsky--or Richard Wagner, the supreme megalomaniac, who not only used multiple tubas but invented a new species (called the "Wagner tuba") for the "Ring" cycle. Some orchestras--the St. Paul Chamber Orchestra, for example, and the Academy of St. Martin-in-the-Fields--have played at the Kennedy Center with no tubas at all--not even a euphonium, which is a sort of tenor tuba.

Tubas suffer from an image problem. Take the case of Tubby, familiar to most children who have access to a phonograph: "a fat little tuba, puffing away, but oh, so slow." All he wanted to do was play a tune like the other instruments, but when he tried, the violins called him a "clumsy fool," the trombone stuck out its tongue and the trumpet snickered. "The poor guy was stuck with the oompahs," says Brian Bowman sympathetically. Bowman is the president of TUBA (Tubists Universal Brotherhood Association), which is holding the Tuba-Euphonium Conference for the 10th consecutive year.

Tubby's story has a happy ending. "All of a sudden, he is given this melody," says Bowman, "and the rest of the orchestra realizes what a beautiful instrument he is." TUBA's objective is to spread that understanding to the rest of the world, and Bowman sees signs of progress. But it moves slowly, as phenomena related to the tuba usually do.

Part of the problem is visual. A tuba player in action seems to be biting the tail of a 30-foot brass boa constrictor that is, in turn, crushing him to death. Most people's impression of the tuba is probably related to hearing a neighborhood kid practicing, and in some school bands the tuba player is chosen not so much for musical ability as for size. "They want someone who looks big enough to carry the thing," says Bowman, "and you get the stereotype of the big, fat kid who carries a tuba. Actually, we now have many women who play the tuba, and some are quite petite."

How do you improve the image of the tuba? TUBA has been doing it by commissioning music for the instrument (there will be more than half a dozen world premieres at Maryland this week), by raising performance standards, by motivating manufacturers to improve the quality of tubas and by persuading people to listen.

"People think we are loud," says Bowman. "Actually, the tuba is not nearly as loud as the trumpet or trombone. It is a mellow, rich, deep instrument. People are not aware that we are really serious about our instruments. But we are dedicated to the idea that a fine musician can express music through any medium he chooses, and there are many fine musicians today who have chosen the tuba and euphonium.

"We are trying to reshape the image of the tuba as the belcher of the band, the musical curiosity, and we are hoping that this will all be changed by the work we're doing. People may come to see us with the idea that we are funny or curious, but once they hear the sound, the musical quality that can be produced by these instruments, the image changes."

So what did you expect him to say?