A license grants you access to plugin updates and priority support, so which license should you get?

Single site license

You should get the “single site license” if you want to use the plugin on a single production site or if you want to use it network-wide on a WP Multisite install.

Up to .. sites license

You should buy a multi-site license if you’re planning on using the plugin on a number of different sites, like for different clients or different business units.

Note that you can always upgrade to a bigger license package later and only pay the price difference.