To me the right number of bikes is the number you actually use regularly, whatever that is. I now have three, my go-to commuter for everyday riding and winter riding, a road bike for fun rides and summer riding, and a mtb with a front shock for trail rides, which I don't do very often but would like to get better at.
I could actually use a separate winter bike, because my commuter gets so much wear on it and breaks down in winter sometimes. But I can't really keep more than three bikes in good working condition. I had a fourth, an old one that was meant to be used as a back-up, but I ended up never using it and re-used the parts elsewhere instead. I would also love a full suspension bike at some point, but that would probably be instead of my now trail bike.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett