This article explains how SPCAF Quality Gate License works
SPCAF Quality Gate licenses (previously called Build/Server) 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 licenses is required for each machine with a build agent.