There may be a limit to the number of resources that you can add to the Project Server Enterprise Resource Pool, but if there is, it is extremely large (more than you have in your company, for sure).
I suspect that something is broken in your system. What exact steps are you taking to add these resources? Are you adding them via PWA or via Project Pro?
I suggest checking the Project Server queue (in the PWA Server Settings area) for errors, and if that doesn’t provide any good clues, then I suggest checking the server event logs and the server ULS logs.
