We usually recommend to have as many "seats" as number of concurrent users that'll work with scenes containing Forest object(s). The license is not assigned by opening 3D Max or any scene which doesn't include any Forest object. Once anyone opens a scene with some Forest object(s), the license is automatically assigned to that computer. The license handling process is fully automatic - the license is sent when needed and released to the "free & available" status when not in use - that happens shortly after closing the 3ds Max (within seconds) / scene with Forest objects (within minutes). If you need to work in "interactive" mode (create, edit, render) with scenes containing Forest objects on several computer(s) at the same time, you should acquire corresponding seat(s).
Hope that helps.