So much of this depends on things like where you live, how and why you ride, how much space you have and many other variables.

Being honest with myself...I legitimately "need" only three bikes...one as a multi-purpose commuter, cargo, do it all bike capable of handling gravel, one fast road bike and one back up/winter bike. Anything beyond that are bikes I want. I could see an argument depending on the purpose and person, for more specialization.

Looking at my collection, the Koga Miyata is clearly my most practical and needed bike. It's also the bike I find myself riding the most often. It can handle rain, gravel, loads, distance rides and some day will be the touring bike I take cross country. I have the stump jumper as the winter/back up bike and then a bunch of road bikes. I also have the International, the stated purpose of which was trail riding and fast commuting, but the Lemond Poprad works better for both.

When looking at thinning my stable, the International is clearly the odd man out. I also sold the De Rosa to help fund the Marnati because I couldn't justify having two Italian Campy road bikes. The Merckx had the same group as the De Rosa (8sp ergo) and the Marnati replaced it as the orange Italian. What I'll probably do long term, because I do want a De Rosa in the collection, is to find a panto late 70s or early 80s model. Ideally I'd want the 1987 De Rosa signature, because that was the first bike I ever truly lusted for. The second was a Merlin.