it varies. there is a "classic" configuration for symphonic orchestras, baroque orchestras and so on. The symphony orchestra is most typically from about 75-90, but...actual results vary a lot because everyone is trying to be interesting (in that classical music "radical is having 100 members of your orchestra! kinda way). The quantity of each of the instruments varies within some general guidelines and in general the world of classical symphonic is well aware when you're pushing the boundaries that are as much about tradition and norms as they are about composer-driven-rules. You'll almost never see anything but the 1st violins to the left of the conductor, but...you'll see lots of variation in the placement of the bass for example and percussion are virtually always in the back, etc.
