On CSS Scan Pro, each license is limited to 3 simultaneous activations.
Activations are by browser and account (not by machine).
You use Chrome (only one Google account logged in), Firefox, and Safari
You use Chrome with 3 different Google accounts logged in
You use only one browser and account on 3 different machines
You can always deactivate or see all your activations, anytime, at the My License page: