I would like to own more bikes but thinking about it more paractically I could probably own less.
1 - commuter - ridden every day
2 - hardtail - night rides + winter weekends
3 - full suss - summer weekends, races etc.
4 - winter road bike - was ridden a lot but likely to be retired
5 - nice new road bike - not ridden yet but probably weekly
I would always keep the first two, the full suss might be replaced with a nice hardtail but I like having a posh mountainbike for bigger days out and racing and a cheaper one that I ride more but clean less (currently singlespeed). I like having a road bike. The only reason I have a winter road bike is because I would get virtually nothing for it if I sold it. I don't ride on the road much during winter so I could happily live with one and the extra cost in maintenance.
It depends on how much you ride as to how you justify the number. I ride 5 days a week to work and occasionally to the shops at weekends. I ride Thursday nights off road. I try to ride at least one day at the weekend, two if I'm lucky in summer usually alternate MTb or Road depending on mood. I also do a mid week road ride in the evening when the days are longer.