It depends on your level of experience, and on what you want to tie. If you're just starting out - as most people asking this question probably are - I would suggest buying between two and four ropes. This will give you the flexibility to tie some basic harnesses, as most harnesses require at least two ropes. An experienced rigger will typcally have 12 or more full-length ropes, plus a few short bits for finishing off ties. It's quite common for experienced riggers to recycle their old and heavily worn ropes to use as short bits for a future set too.