Depends on which orchestra I was in actually

School band was either two (highschool) or three (juniorhigh).
In the actual orchestra I played in though - the EYO (Edmonton Youth Orchestra) - we had 8 horns total. Four for each division (senior orch/junior orch), though occasionally we would shuffle people around as the need arose. Getting to play with the senior orchestra was pretty damn cool (if indimidating and extremely difficult!) for us.

Now, at band camp... oh babeee! We had like 20 of us running around, plus teachers. One day we made copies of some 16th century or whatever (all I knew is it was OLD) honest to god hunting-horn music, grabbed a bunch of stands, ran out into the courtyard (this was on a college campus)... and just belted out these hunting tunes for like three hours!
Now that was an experience i'll never forget. We had so many horns playing so loud, you could literally hear the music echoing off distant buildings. Crowds gathered, people stared, and later on we got in shit from other teachers about "making such a racket".
All I know is that felt and sounded so very, very fucking cool.