This article explains how a build license works.
SPCAF Build licenses are licensed by the number of servers and the number of CPU cores. A server is a virtual or physical machine running one or more build agents using SPCAF.
e.g. If a build controller has 2 or more build agents on the same machine, one license is sufficient as long as it covers the amount of cores. If the build agent is on a separate machine, a license is required for each machine with a build agent.