I do not know the limits but actually, the scenario format is a simple text file and the structure of a mission (viewing in a text editor) is quite understandable. So you could just try to copy&paste units/triggers (in the text editor) and give them just other names etc. to check the limits of the engine.
If you have a big scenario/campaign file, it might even be quite handy to alter sometimes data directly in a text editor later.
May I ask what your campaign is about ?
