How can I know how many people have filled my form?

Can I keep track of the number of people signing up for an event so that I do not oversell seats?H

I suggest that you set a limit for the submissions that can be accepted in your form, so you won't oversell seats for your event. You can see how to do it here: http://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form

You can also use our submissions counter widget in your form to show how many seats are being taken or how many are available (if you set the submission limit in the form).

Let us know if this helps.
Thanks.