Licenses for DEVONthink 3 and later can be used on up to two Macs. When you enter the license code into the app an identifier for your computer is added to the license, occupying one seat. You can check on which devices a license is currently being used in your customer account. Here you can also remove devices from a license to free up the seat.
If you have more than two devices you can buy additional seats from your customer account.
Older licenses can normally be used on all computers that you own and operate yourself. They are not valid for e.g. a separate account on your Mac used by someone else.