One mountain bike – an Alfine/SS steel HT. Suits all my riding and I’ll take it anywhere. Not the lightest, not a lot of travel on the front, but it’s a lovely ride. I’ve ridden my son’s FS, but I prefer the HT.
Road bikes, however. Well I have a Super Bike, if that’s what they are called. I’ve raced it, commuted on it and sportived on it. But I still need two others – at least 😉
A fixed wheel commuting road bike with proper mudguards and a rack for normal day-to-day commuting and wet training
A winter and race bike for salty roads,bad weather and crash fest crits!
(A cyclocross bike may be a fourth, and a folder is a nice to have.)
So for road, no chance. For off-road, a super-bling custom singlespeed would be enough for me (Niner SS carbon, probably) because that suits my riding.