Each plugin comes with a license. You will see the options on the general pricing page and in the pricing section for each plugin page.
Two types of licenses define how many "production" or "live" servers the plugin can be used.
Single-server license
Previously called "single-site license", allows for one "production server" installation.
Multi-server license
Previously called "multi-site license", allows for an unlimited number of installations within one company.
For every additional company, you have to purchase a separate license, despite the number of sites you are allowed to use with your current license.
Testing and evaluation
You can have multiple "testing" installations for evaluating and testing purposes. That means that with a single-server license your developers can still install the plugin onto their local machines without any limits. However, only one "production" installation is allowed.