well you need a back up QB in case your starting QB gets injured, but every position really need a back up. and there are many different formations which call for different personel
a starting offense has in, for example, an i-form has:
1 qb
5 lineman: 1 left tackle, 1 left guard, 1 center, 1 center, 1 right guard, and 1 right tackle
2 running backs: 1 halfback and 1 fullback
1 tight end
and 2 wide receivers
there are always 5 lineman and a quarterback, the number of tight ends, wide receivers, and running backs change by formation