Most musicians began playing in 5th grade when they could join band or orchestra. String players sometimes start at a younger age. To be this good, musicians have to begin at age three. They are so good that they never practice.
Musicians practice their parts alone, never together until the concert.
It depends on the stage - as many as can be fit on stage without falling off. It depends on the music - generally 70 to 85 musicians, but some pieces of music require more instruments, others less. It is hard to find good tuba players and easy to find lots of violins. The violins often play the main melody and need to be heard. It is more fun to watch a bunch of violinists bowing than a group of tuba players puffing away.