Depending on the ballet, it could be an entire symphony orchestra, or a single guitar. It all depends on the choreograhers vision of the ballet. It is mostly the piano that you can hear.

most of the time ballet dancers dance to the orchestra ------------------------------------------------------------------------------------------------------------------ in class it might just be piano, but the music for shows is almost always ochestra

a lot of people played the piano in the orchestra

A dance (ballet) accompanied by music played by a piano only.

None of Beethoven's symphonies has a piano part. He wrote five concertos for piano accompanied by orchestra, and a Fantasy for piano, chorus and orchestra.