I think having one bike for every kind of riding you do, plus a few bikes for fun is just fine. Once you end up with bikes that don't get ridden for 6 months, you probably have too many. I think when you have more projects than rideable bikes, you have a problem. I definitely have a problem.

Rideable:
92 Miyata 714: main commuter
94 Trek 850: Snow commuter/MTB
Tall bike!
Giant Butte: uber beater locked outside at work
60 Hercules 3 speed (needs some love)

Projects:
78 Maruishi Roadace 707: building a fixed wheel next week
01 Specialized FSR Enduro Pro: Need a rear shock
Leader something or other: This will be my fast roadie, in case I decide I want to race
89 Cannondale MTB: will probably be a SS cross country
70 Stingray Fastback- need a 5 speed stick shift ($$$!)
7? Peugeot U08- may replace the Trek as a snow bike.
24" Rat rod/low rider project
Jamis Eureka: may never get built now that I have the Specialized and the C'dale, maybe for the wife.