Final Draft comes with an unlimited number of installs but the retail license allows two computers (that you own and operate) to run at a given time. If you have (for example) five computers, FD can be installed on all of them and the two activations moved from computer to computer as needed. Deactivating a computer returns the activation to our server and the activation can subsequently be downloaded onto another machine. Deactivating leaves the installation and all of your Final Draft scripts on the machine and fully usable once the machine has been reactivated. Information on deactivating is here: How do I deactivate my software?
Multiple-user site licenses allow for X number of computers to run Final Draft, per the terms of the sale.