For the Pro edition ($995 USD), you need 1 OSForensics license per machine that is running the software, or per device that has the software installed at the same time.
For organizations that carry out large numbers of overlapping investigations and thus require more flexibility, a site license is recommended. Site licenses cover an unlimited number of users and devices within a single organization within a single country.
Example #1: You have OSForensics installed or imaged to a USB Flash Drive and you plan to use it to investigate several machines but at different times. Only one license is required.
Example #2: You wish to use OSForensics on two machines at the same time (either by installing OSF to the machine or having OSF installed or imaged to a USB Flash Drive). You require two single user licenses.
Example #3: You have 5 staff in the field investigating PC's. Each investigator carries a copy of OSForensics with them on a USB drive. 5 licenses are required.
Example #4: You have several machines to investigate using OSForensics and you wish to do so by installing OSForensics to each machine. Only one license is required as long as OSForensics is uninstalled from it's current machine before installing it to the next one.
Example #5: You are part of a large organization that employs 50+ investigators within the same country. Each investigator requires their own copy of OSForensics on a USB drive, plus more to install on their work computers to investigate saved images. A site license would be the most economical purchase.