When buying the PRO version you can see how many licenses you will get.

This refers to the number of active licenses you can have running at the same time. For example, if your license includes 5 activations, that means you can use it on 5 websites concurrently.

You can reassign the license to different domains as often as you like — so if you only need it active on one site at a time, you can have one single license and you can switch it between projects freely. Just keep in mind that you can’t use it simultaneously on more sites than your plan allows.

Note that for multisite setups, each sub-site is considered its own website, as the domains are different. You can use a single license, but don’t forget to change the domain each time.