Restrict Content Pro – Limited Quantity Available GPL Overview:

This free add-on enables you to limit the number of sales a subscription level can have. Once the total quantity available for a subscription level has been reached, the level is deactivated so that it’s hidden from the registration form. Existing subscribers will continue to be billed for the subscription if they’re set up for automatic renewals.

Limited Quantity Available is easy to use. Just define the number of times you want the subscription level to be sold, and everything else is handled automatically.