The software can be installed on any number of machines. Its the iLok(with license) that is needed to actually run the software. You can carry the iLok around with you as you move from one system to the next. Otherwise, you would need a license and iLok for each machine if you need them to run at the same time.
Some random thoughts:
The iLok can move from Mac to PC and back without issue
You can have multiple iLoks registered to the same account
You can connect multiple iLoks from different accounts, to 1 computer
Working with a laptop-I highly recommend using a "dongle buddy"(short USB cable) to avoid the risk of bumping and breaking an iLok that sticks out(laptops often get moved around)
Moving an iLok constantly puts it at a higher risk for failure/damage so buy the ZDT coverage and keep a spare iLok around(sometimes you can get a free iLok with certain plugin purchases).