Paypal form selling - setting limits to how many can be paid

I am running a site that does contests. I am using your form as a way to collect information and an entrance fee. I need to find a way to limit the amount of contestants/ entry fee's purchases. (basically making a stock amount of entry fees, once 200 are sold, you can't buy anymore. How can I do this? Thanks for your help. 

-Sean

Hello Sean,

Thank you for contacting us.

To limit submissions in various ways - please consider the methods outlined in this linked guide.

Let us know if this helps.