I always have 2..my mtn bike with slicks/knobbys depending on commuting or trail riding and a roadbike. right now it is a cannondale supersix.

Over the past year I've bought/sold over 10 bikes however...but I always need to have 2 (daily and a weekend bike). Lets me ride lots of different stuff and slowly improve my own gear. I usually pour X amount of money into a hobby and through the magic of craigs/ebay get to have a lot of different stuff as time goes on