What is my license activation limit?

This depends on the plan you have purchased. For example, if you have purchased the starter plan (1-2 sites), you can install and activate your license on up to two different sites. If you have purchased the unlimited plan, you can install and activate your license on an unlimited number of sites.

Multisite activation Limit

As for single sites, the Multisite follows the same rule. if you have purchased the starter plan (1-2 sites), you can install and activate your license on up to two different WordPress Multisite installations.

To be more general, each WordPress installation (either normal one or MU one) should have one license active.

If you have separated WordPress installations (either a normal installation or a MU), you should have a license for each installation even if those installations are under the same domain name.