THEY'RE turning loose a monster out at the University of Maryland on Thursday. That's the night the university's 11th Annual International Piano Festival and Competition is holding "An Extravaganza for Multiple Pianos." The stage of Tawes Theater will be littered with 10 concert grand pianos on whose 880 keys thirty pianists will be working out. That means 300 fingers will be tearing back and forth over 360 black and 520 white keys. Oh, not all of them all of the time. But some of the time, wow!

And think of the poor pedals. Did you know that there are pianists whose feet come down on the pedals so hard you can hear them all over the hall? Others raise one foot clear off the floor and then descend on the pedal from a 45-degree angle that gives a special body english to the music and more anguish to the pedal's sacroiliac. Still another school of pedal-thought prefers the subtler but equally painful approach of placing both feet carefully on top of two pedals at once and pressing down slowly until the entire weight of the pianist is mashing the pedals almost to the floor. Is it any wonder that the history of piano recitals is scarred with accounts of pedals that fell off the instrument, or crashed backwards through those supporting diagonals, or simply went limp and refused to lift one more felt hammer off another string?

But back to the extravaganza. There will indeed be times when all 30 pianists will be huddles in groups of three in front of each of the 10 pianos. There also will be lots of times when a mere 20 pianists play at a time.

"What in the world are they going to play?" you ask in open disbelief.

Well, for starters, if seems that Carl Czerny -- wouldn't you know it would be that inventor of special exercises devised for the tutoring of young children? -- arranged the overture to Rossini's opera, "Semiramide," for practically any number of pianists to play on as many pianos as possible. (Czerny studied with Beethoven, and there are those who maintain to this day that Beethoven hated the piano. That would explain a lot about Czerny.) Not to be outdone by Czerny, along came John Kirkpatrick a century later and arranged the second movement of a symphony called "Night in the Tropics" by Lois Moreau Gottschalk for lots of pianists in front of lots of pianos.

Gottschalk is going to be very big on extravaganza night, because Eugene List, who is an old hand at setting up these things, is going to be the soloist in a bacchanalian Grand Tarantella by the same Louis Moreau. And just before the intermission, there will be one of Gottschalk's most famous pieces, "The Union, A Grande Fantasie on Patriotic Airs." Since L.M.G. lived at the time of the Civil War, you can imagine how many songs of the Grand Army of the Republic will be heard in that number.

"Heigh-Ho, Silver, Away!" will open the second half as the overture to "William Tell," also by Rossini, reaches the most famous of all operatic finales, and you know by now how many fingers and toes that one is going to require. And guess who arranged this one? Gottschalf! In between these monstruous outbursts, however, there will be moments of surcrease in which only one, or perhaps as many as two, pianists will be heard at a time. After all, some of the loveliest music in the world was written for only four hands on only two pianos. Like the "Arensky Waltz" and Bizet's "Children's Games." And "Scaramouche" by Darius Milhaud. These and even some solos will break up the giant monopoly of those 10 black and white keyboards.

But inevitably the 300 fingers will move in again for things like the prelude to "Carmen" and one of the Hungarian Dances by Brahms. What seems strange is that Sousa's great march, "The Stars and Stripes Forever," arranged by Morton Gould, is going to use only 20 pianists. Apparently no one ever told Gouild he could have 30! Yes, Virginia, it indeed does take a conductor to keep so many pianists together, especially when they are making so much noise. Three will be on hand on the 23rd: Alceo Bocchino, Silva Pereira and Pierre Colombo. The whole thing will start at 8:30 p.m.

(In the midst of all this, take a moment to pity the poor piano technician. Ned Dodson, the official tuner for the Maryland Festival, says he is not going to bed at all the night before the Monster Rally. It takes long hours and lots of coffee to tune 10 pianos to operate together.)

The starting time for all the evening recitals being played during the week-long festival and competition is 8:30. Tonight, Gyorgy Sandor will play a Bartok program marking the composer's centennial. Tomorrow night Lili Kraus will play Haydn, Mozart, Beethoven, Bartok and Brahms. Tuesday brings Rudolf Firkusny in Haydn, Schubert, Beethoven, Debussy and Martinu, with Israela Margalit on Wednesday playing Bach, Debussy, Chopin and Prokofiev.

On Friday, if there is still a piano in shape for another recital, Paul Badura-Skoda will play Bach, Schumann and Chopin. The finals of the competition will be played on Saturday beginning at 8, with Pereira conducting the Baltimore Symphony for the three finalists who emerge victorious from the field of 40 who entered the preliminaries last Friday. Twelve of those will compete in semi-final sessions to be played daily, today through Friday. All of these events, including the 10-piano extravaganza, are open to the public; tickets at the Tawes box office. So, to, are master classes at 10 a.m. July 19 through 25, and 1:30 p.m. lecture-recitals July 20 through 25. If you have any questions after all this, call 454-3347.