I could probably make due with one, and definitely two handguns to address all of my "needs." Like most here though, I choose not to.
Hunting: I don't hunt
Competition: I don't participate in any
That leaves, range/plinking, home defense, carry (only occasionally since I live in MD, a couple times a month when in VA, DE, or WV and carry on my non-resident UT permit), and camping/woods gun.
If I was to narrow it to one or two guns I'd narrow caliber first to 9mm and/or .38spl/.357mag. Both are relatively inexpensive for range/plinking (though with .38spl, long gone are the days of nearly 9mm cheap practice ammo, heck, 9mm isn't cheap like it used to be either, but with reloading .38spl is easy to reload and inexpensive). Both are versatile and can be quite mild for extended practice or teaching new shooters, and both have great self-defense loadings, plus the .357mag options that go to heavy duty woods guns loads as well. As for woods/camping loads, 9mm should do it most of the time. I live in the mid-Atlantic and do most of my camping here. Most bears are man-sized (a 300LB black bear would be big around here) and a 9mm (especially with some of the heavy woods loads by some of the specialty ammo makers) should do it, usually bear spray is better than a gun for bear defense anyway, and most black bears are shy and avoid people, but there are always exceptions so a nice .357mag might be nice to have.
Short answer:
I have a few options I may look to:
For 90% of my use (range/plinking, home defense, CCW): CZ PCR/P01 (I have both). If I would ever limit myself to only one, it would likely be one of these.
For when I need smaller: SIG P290RS. After the current gun availability craziness has run its course I may replace it with a higher capacity and lighter P365 or and/or a much lighter Kahr PM9 (though I'd possibly keep the P290 as well, though I'd likely never carry the P290 anymore and it would no longer cover a "need")
For when I want/need a .357mag: Colt King Cobra (new 3" model)
With those three, I really have all my "needs" covered, and most of the time I could skip the Colt and see the vast majority of my "needs" covered with just the first two (or skip the CZ and go with the second two). Still, it is nice to have other niche uses covered as well with guns that more specifically fill certain roles.