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 for each license you need. When you get to the Checkout, you'll see an option to add more licenses. From the dropdown, you can select from 5, 10 or 20 site licenses.
If you want all of your 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.