I've never attempted a model with more than 10 floors. If you send the files and instructions needed to reproduce the crash to support@thunderheadeng.com, we may be able to figure out what's going on.
I've already added a feature to the next release that will allow you to move objects between "floors" (in terms of the tree), so we should be able to get back to a 21 story building having 21 floors as soon as that's available.
When you say importing 50 floors, do you mean using the Extract Floors tool 50 times, or do you mean using the Import action for part of a DXF/FDS file?