Owning a copy of Capture One Pro entitles you to using C1 on two computers.  With the release of Capture One 4.1, Phase One has simplified the system that tracks and manages your use of Capture One installations. You can deactivate a particular installation from within Capture One or manage the keys remotely using your Phase One Profile at phaseone.com.

See the links below for various articles on Capture One user licenses:

All about licenses: Upgrades, Updates and Activations Explained
How To Activate Capture One
Where can I find my Capture One License code?