It was not necessarily number of layers but the amount of system resources getting dangerously low. Windows will crash when it wants to. What you can do when working for a while on an image, Edit - Purge and select Undo, Clipboard, Histories or All. You can merge certain layers that don't need to be on separate layers anymore. Edit - Preferences - General - Reduce the amount in the history states - Memory & Image Cache - Check your cache levels and the percentage of memory used by photoshop. If running other programs make sure you open photoshop last. Always save your work from time to time if you don't want to risk losing it by a crash.