Buying more licenses for new project (Exchange Migration Manager)

Im migrating a serveral diffrent departments (same corparate group) in to a central mailsystem, first company is done and we used up all the licenses, i have now created a new project for the next department (all have theire own mailsystems)and bought new licenses but they are already used by the old project? Im i doing something wrong when adding the licenses? Seems like the licenses does not stack (e.g. 100 licenses and then buy 50 more i would say i have 150 but it says that i have 50)