I've currently got 6, but am in the process of (re)building another:
NSbikes Society, XCish hardtail with Pikes
Muddyfox jumpbike (old, singlespeed)
Ribble 731 roadbike with 16speed campag
'97 Condercone with drop bars, mudguards and slicks for crappy weather commuting
KHE BMX for playing at skateparks and wallrides
Kona Howler 4x bike, 4" travel, getting more like a DH bike as it's now got boxxer 151s, 8" rotor and big tyres
As I'm currently turning the 4x bike into a DH bike, thought I'd better actually get one so I'm rebuilding my 1998 Schwinn Straight8… Got some saint hubbed wheels from here and with some careful machining they now work with the 20mm/15mm stepped rear axle.
Only thing I'd like in addition would be a 4 or 5" travel XC ish boingy bike – would need to be laidback angles. Might turn the howler into this in the future.
Oh, and possibly another singlespeed as I sold my last one to buy the NSbikes frame…