I have 5 bikes at the moment, 3 ready to ride and 2 being rebuilt. I commute but also ride on the weekends so I try to keep my stable mixed and then modify the bikes to best fit their usage. I also keep some crossover abilities so if one is down, I have a good backup.

Current
1) 29er MTB - dedicated offroad only
2) 80's Peugeot MTB - Daily commuter(Fenders/Rack/Slicks)/Intermediate weekend rides(10-20 miles)
3) Fuji Touring - Longer road rides(>20 miles)/hard packed trails/Commuting

In Progress
1) 70's 10 speed - Going to be a city bike. Porteur rack/Upright bars and riding position/Shopping bike/Commuter
2) 80 Trek 400 - Eventually will be my lighter long distance bike. Will be doing some Century rides.

All of this and yet I still need one more bike. I'm going to add a single speed MTB to the stable at some point. I'm just waiting to grab the right bike of Craigslist.