Starting with v6, a single user can run ClipMate on his computer plus one other computer that he/she is the primary user of. So if you have a desktop and a laptop, then you can use the same key twice. But if you have a desktop and a seceretary with a desktop, then you would need to license each separately.
If you have multiple machines on a LAN and you are the primary user of each, you can only use the extra key on one of them. So you may still need to purchase additional copies to run on more than two PCs. You can use the multi-user site license pricing though.