I agree it's what's best for you. Some folks wouldn't bat an eye at paying $5k for a photographer, and some balk at it. ::shrug:: it's all relative eh?
The ladies above made some great points for when it comes to the "big" stuff.
For smaller stuff, invites, gifts, favors etc. comparison shopping is super important because there's a wide margin on prices for the exact same product.
I'd say before you decide on number of people, you should find the budget that works for you FIRST. Good luck and happy planning!