I've got some cities with a lot of buildings in them and haven't encountered a problem with a limit on that yet.
Building heights beyond 6 floors can cause problems though. A 6th floor can be made without even cheating. Just place a dormer on the 5th floor, start building, then delete it. But beyond that, especially with higher numbers, you may get slow loading, jumping, even crashing. It's nearly impossible to get the camera to move right to furnish anything above 8-9 floors and you can really only look at the whole building in neighborhood mode. I think that's the only reason some people even build them is neighborhood decoration and they don't intend to go into the lot.
