There is a program that finds the hacks and lists out which ones will interfere with your game:
http://www.simwardrobe.comIt is called "Sims 2 Hack Conflict Detection Utility" and it detects the conflicts between similar hacks.
There is no limit (that I know of) that the content manager has-it is the game's processors which determine the amount of stuff you can have. For instance-if you have a napping hack from one site, and another from a second site, that will conflict. There are many instances of conflicts with some hacks, and you have to be careful which versions you have.
You should read the manuals for every hack you have, and note if they conflict with other hacks-most modders will list the conflicts if they know of them.