I agree, I'll specify the number. Do you think 400 would be good/enough? My only intention is to make it fair for both sides. I doubt any project would need more than 400 icons. Unless its purpose is to distribute icons!
I suppose it depends on how complex a model you're willing to support. There might be some pricing opportunities available to you here, where a sliding scale can accomodate projects of various size. Of course, I could be overthinking the whole thing too.