When we say there's a limit to the "activations" we mean "allotted activations". It's hard to get this point across on the signup page.
For instance, the Solo plan allows 300 "allotted activation". Every product key you generate uses 1 (or more) allotted activations. So if you use a product key to activate and deactivate a million times, you're still only using 1 allotted activation. Does that make sense?
You can view the total number of allotted activation for your account on your account page.