You need 1 license per machine that is running the software, or has the software installed at the same time.
Example 1: You have a PC production line that produces 10 machines per day. You test each machine during the night. So a maximum of 10 machines are running the software at any one time, and you re-image the machine before shipping. 10 Licenses are required.
Example 2: You are a company that takes delivery of no more than two new machines at a time. You want to burn in both machines at the same time. 2 licenses are required, assuming you uninstall the software before putting the machine into service.
Example 3: You have 5 staff in the field repairing PC's. Each technician carries a copy of BurnInTest with them on a USB drive. 5 licenses are required assuming the software is always run from the USB drive.
Unlimited usage: Site licenses are available for coverage for an unlimited number of users within a single organization within a single country.