Currently I have three: road, 'cross, and mtb. I had two other road bikes, one of which I gave away to one of my students, the other of which I sold to a 12-year-old boy for $100 (it was carbon fiber and he wanted to race). I also gave my first mtb (Voodoo Hoodoo hardtail) to my then-boyfriend in Nicaragua, who proceeded to lie and steal from me; now he uses the bike for a bike-touring company he just started with his new Dutch wife. At least the bike went to a good cause.
As I get older, my cycling changes and I don't want such aggressive positioning on my road and 'cross bikes, and a hardtail mtb is too punishing in my new geographic location. Now I ride a classic steel road bike (Gunnar Sport), an old, steel 'cross bike (Surly Cross Check), and a full-suspension 26" aluminum mtb (Giant Reign).