1. for enjoying music on my computer rig (open, circumaural, sennheisers)
2. for playing games on my computer rig (closed, circumaural, currently AT)
3. for sound analysis on my computer (closed, circumaural, currently Ultrasone)
4. for portable/mp3 use (canalphones, currently etys)
5. for main stereo use, occasionally during evenings (open, circumaural, sennheisers)
6. for my wife for listening on the computer (closed, circumaural, beyers)
7. for my wife for listening on the go (supra-aural, senns)
8. for cheap on-the-travel listen (canals, currently philips)
Yes, I know... too many.
Do I really _need_ them all. Of course not.
Do I benefit from having and using them, yes, very much so.
Still, I think at least three pairs is a minimum I need (portable, closed and open).