Ubisoft forum manager bukowski113 posted the deatils on the game's official forum:
# You have 5 activations on 3 separate PCs.
# Uninstalling the game "refunds" an activation. This process is called "revoke", so as long as you complete proper uninstall you will be able to install the game an unlimited number of times on 3 systems.
# You can upgrade your computer as many time as you want (using our revoke system)
# Ubisoft is committed to the support of our games, and additional activations can be provided.
# Ubisoft is committed to the long term support of our games: you'll always be able to play Far Cry 2.