We currently do not support this. There are no limits. If you think others would be interested in this feature post it to the Idea area so others can vote on it: http://forums.adobe.com/community/formscentral?view=idea
PLEASE add this feature. as far as I can tell there are no option out there that serve this need. FormAssembly, EmailMeForm, or FormsCentral. Think of how many customers would use this. Imagine a t-shirt company that pre-prints inventory, wouldn't they want to limit how many forms could be submitted that want a size L shirt? My business is participant based, like the evnt question above we need to limit the number of participants. Of course I could program my own solution, but then I would miss out on all the other cool FormsCentral features. Any chance this could be added right away if I sponsored it as a custom development?