iirc it depends on the version. In vanilla you can only have 16. In PTW 24 and in C3C you can have 31. It use to be you could only have 4096 units in C3 and PTW. I know they increased it to 9192 or something like that. I know there is a limit to resources and other things, but do not recall the values.
In the creation forum in CFC I have seen the numbers, but not sure how easy it would be to fine the info.