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What instruments and how many in each section would be called for in an average sized composition for a symphony orchestra? So really just the standard instruments and a general idea of the number of members in each section.

Kind of a loaded question since there are so many possibilities, but in general:

3 flutes, 3 oboes, 3 clarinets, 2 bassoons (+ optional contrabassoon), 3 trumpets, 4 horns, 3 trombones, tuba, 3 percussionists, timpani, harp/piano optional, and (of course) strings.

Standard for what purpose?

I'd say that the base setting is two of each woodwind+2 or 4 horns, strings and percussion. (Everyone else will meet plenty of music where they won't have a part.) Additional brass is not uncommon, but I hesitate to call that standard. For everybody except the strings, one player per part is the normal thing.

Look up some sheet music to get an idea of the usual variations. The Werner Icking Music Archive is a good place to start.

Edit: I forgot that WIMA is merged with IMSLP. Go there instead. For example, here's Mozart's 40th Symphony.

There're several incidents where I know a piece will travel with a reduced orchestra; sometimes, depending on for whom you're working, you may consider more modest forces.

I often have to consider 2,1,2,1,3,3,3,1 for winds and brass. You can do a lot with that, believe me.

Such forces, depending on context, can help get a performance of your work more easily.

YMMV

J.c.S.