This number was way up at one time and now is down. So now I look at how many bikes I actually ride on a regular basis. I suppose my criteria is do you use this bike for its intended purpose (e.g. road, trail, mtb, race, groceries, commute) more than once in a great while.
So I come up with 9 by that count:
road classic for tours (2)
road race (1)
time trial (1)
cyclocross, commute (2)
cruise to coffee/ice cream/newspaper (1)
fixed gear summer (1)
fixed gear winter (1)
This is just me. For my wife the number is 3. For most of our kids it is usually 2.