is there a way to limit how many people can submit and pay per form.

You can control limiting the amount of submissions allowed to be filled in by a specific user by configuring the Form's Unique Submissions which can limit how many people are allowed to fill it out. However, unfortunately we do not have a feature to fully support this ability. It may be possible to do so via the form's source code but this would require custom programming knowledge if you have some or know someone who does this and it would have to be done outside of jotform.

You can also read a similar answer on this found here: http://www.jotform.com/answers/95277-Auto-updating-number-of-available-spots since as of yet we do no have any updates on a feature for this.

Hello Anja.  On your form, you can click on Preferences > Form Limits tab > Change the value for Submission Limit

This way, you can limit the number of submissions your form can receive.  After the number of submissions limit is met, the form will automatically disable.