A horde of pianists will descend on the Maryland University campus next week for the opening of the 10th annual International Piano Festival and Competition. Thirty-five musiciians are coming from 13 countries and 29 states to compete for the nearly $15,000 in prizes that will be awarded on Saturday night, July 26, at the close of the competiton. The first of 15 cash prized is $5,000, in addition to which the top winners are assured of orchestral and solo appearances as a result of their playing.

The 35 entrants in the competition will be heard in preliminary rounds starting at 9 a.m. on Friday, July 18. Three rounds will be played on Friday and Saturday, reducing the number of semifinalists from 35 to 12.

The competition, conceived and directed by Stewart Gordon, who is now the head of Maryland's Music School, has recently been given membership in the International Federation of Music Competitions at Geneva, which is a kind of center for such contests. Judges for the competition are Idil Biret, Leon Fleisher, Valentin Gheroghui, Nataline Hinderas, Tikhon Khrennikov, Adrien Meisch, Silva Pereira, Walter Ponce, Walter Robert, Thomas Schumacher, Valerie Tyron and Beveridge Webster. For contest watchers, the daily sessions in which the 12 semifinalists will be narrowed down to the final three will take palce daily from 3:30 to 5:45, Sunday, July 20 through Friday the 25th. The finals, with the Baltimore Symphony under Fleisher's baton, will be on Saturday night at 8:30.

As usual during the festival-competition, there will be nightly recitals in Maryland's Tawes Theater. These will be played at 8:30 on Sunday through Friday nights by Idil Biret, Gary Graffman, Valerie Tyron, the Beaux Arts Trio, Paul Badura-Skoda and Ruth Laredo.

Among the features of past festivals have been the lecture-recitals offered each day. This year at 1:30 these will include such matters as the Chopin Etudes, Brazilian piano music, 18th-century keyboard music on a Mozart fortepiano, the piano music of Ezra Laderman, the teaching of Artur Schnabel and the piano music of black composers. Masters classes are also offered each day. These will be by Fleisher, Robert, Graffman, Webster, Badura-Skoda, Laredo and the Beaux Arts Trio, who will discuss the piano in the world of chamber music.

Members of the public can register for any of the events or can purchase tickets for the evening recitals. For full details on the festival-competition, call 454-3347.