I've never used CCMagic, so I can't speak from firsthand experience. Some people swear by it; others who have it can get caught in a long cycle of testing and retesting their cc without figuring out what's wrong. Personally, I'd say if you don't already use it, you probably don't need it at this point, although you might decide one day that its features are convenient. If you do try it, make sure to back up all your cc, or your entire Sims 3 folder in Documents\Electronic Arts, so you can uninstall CCMagic and put your old content back if necessary.
There's no hard limit to custom content in Sims 3, although the game starts to bog down with too much. That limit seems to be around 10 GB on newer computers, maybe less on older ones or on slower hard drives, but there's a lot of variation in performance and in what people will put up with to use their cc.
More importantly, even one bad cc item can cause crashing, lagging, graphics weirdness, etc., so it's much more important to test your cc and make sure it's working before adding it to your main saves. One strategy is to install any new items into a clean Sims 3 folder (moving your regular one somewhere else first), use the items in-game in a new save, and look for problems. If there's a problem, you'll have found out before your existing saves are affected.
