I have 4:

2 road- the first generation was a second hand C-dale purchased from a friend which has been upgraded to a C-dale SuperSix.

2 mountain-a 26 inch wheel Titus and a 29 inch wheel Salsa.

Most people that mt bike a lot, or race have 2. My DH just cracked his frame and it tokk 5 weeks to replace. He missed 2 races because he does not have a back up any longer. Needless to say, we will be getting a second bike for him sometime soon.