Multiple Websites

I'm assuming you mean purchase a separate Gravity Forms license for each site. It all depends on how you want to manage your sites moving forward. If you're purchasing on behalf of a client, we usually recommend that you purchase a personal license for each client. That way, the client will have their own support login for the future, they can manage their own renewals annually, etc.

You can always purchase a developer license to use on multiple sites, but the developer license only has one support login/account associated with that key so if you were to use that key for client sites, you would be responsible for providing support for them and would be responsible for keeping the key renewed, etc. If you're just using the key on several of your own sites, then the developer license would be the easy choice. It's really up to you.