FormsCentral doesn't provide any way to limit registrants. You would have to monitor the submissions and close the form when the maximum has been reached.
We do have a Feature Request page where you can add or vote on feature ideas:
This idea matches what you wanted: http://forums.adobe.com/ideas/2380
A similar idea here too : http://forums.adobe.com/ideas/2220
If you need to add a new idea, please go to http://forums.adobe.com/community/formscentral and click on "Submit Feature Request" link under Resources section.