Let's just say it's on the list as an instrument available for use in the orchestra. However, the piano is not used on every single piece of written music for the orchestra! The piano will be used...
One or two. Usually one. Actually, if there is a piano, it is a guest artist and not a part of the orchestra. There are no pianos in a symphony orchestra
Yes, but only when the score calls for it. The composer of the piece determines which instruments (and how many of each) are required for the playing of his/her piece.