For a definitive answer to this, you must really look at your particular license agreement.
However, there are two answers to this question, depending on which license you have, student or individual.
Most purchasers purchase individual licenses. These are of unlimited duration and are licensed for single use. Because we understand that there are different usage models depending upon your needs, we want to be as fair as possible to all of our users. As such, individual licenses can be treated as a per-user or per-computer license.
In per-user mode, you are allowed to install the software on one desktop and one laptop computer (technically, you could install it on a desktop computer in your office and a desktop computer at your home, but this is becoming a less common scenario). The requirement is that it only be run by a single user at any time. This means that if you choose to use this option, you are bound by the license to not allow a co-worker to use the Cartographica with this same license.
In per-computer mode, you are allowed to install the software on one computer, and any one who is using that computer has the right to use the software (note: don't try to get sneaky here, you are never allowed to run more than one copy of Cartographica using the same key at the same time).
The usage models are pretty simple: in a lab-style environment, you can install per-computer and anyone using the computer can use Cartographica. In an environment where the computers are assigned to individual users, you can do the same for Cartographica, thus allowing a single user to have a copy on a secondary computer for work on the road
NOTE: Absolutely nothing here supersedes the license agreement between you and ClueTrust and under no circumstances are you allowed to run more than one copy of the software using the same key.
Student licenses are limited-time and limited-mobility. You are allowed to install the software on a single computer. At the time of installation, it will be node-locked to that computer and you will not be able to install a copy on another computer without contacting ClueTrust to deactivate the software on the current computer and reactivate the software on another computer.
If you have a special license with ClueTrust for Cartographica, please consult that license. This would include site-wide licensing and limited-time licensing.