how many parts can SC handle?

I'm assuming that you mean how many parts in an assembly. The SpaceClaim employees can correct me if I'm wrong but I don't think there is a number limit. It all has to do with how many parts your computer can handle. There's a point that if you get to many parts that your computer will not function any longer. Related to this would also be the complexity of the parts. Or said another way the number of parts could vary dramatically from one SpaceClaim user to the next based on how complex the parts are and how powerful your computer is.

I have not yet learned about a limit and can tell that a few thousand are no problem. I have a eight cores and lots of RAM. It may slow down your machine or rather your graphics once the model gets too complex, though it will need quite some load until this gets apparent. Part quantity is not the only factor, I'd rather say model complexity. While a complex model like a very detailed car body made up of only a dozen parts may have effects on speed, handling a few thousand simple cubes may be no issue.