Each Wp Tasty plugin license is valid for only one site. If you add a single license to multiple sites, it will only activate on one site.
If you have multiple sites that you want to use the plugin on, you can purchase as many licenses as you need. Simply go through the checkout process again for each license you need. If you want all the licenses to be associated with the same WP Tasty account, simply log in to your account before purchasing, or use the same email address you used for the first purchase during checkout.