It all depends on the type of symphonyorchestra you are talking about and the time period of the symphony orchestra. Classical Orchestra - 12 violins Early Romantic Orchestra - 26 violins Late...
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
it usually depends on the type and size; Usually one, sometimes 2; I have played where I'm the only harpist, and other times where it is a harp enselmble or orcchrestra where everybody plays the harp.
Depending on the piece of music - if baroque era, 4 - if classical era, up to 8 - if romantic era - up to 12, if impressionist era - any amount, if modern era - any amount. Generally speaking, since...