Variety is the spice of life so I shoot 9mm, .40 S&W, .45 ACP and .22 regularly.

45acp here.

I think .35 Cal is the best diameter for a bullet. Very versatile. I honestly believe .380, 9mm .357 will do everything a normal person would need a handgun to do.

I don't own a .357 yet, but 3 9mm, a .380 and a .38spl and I'm cool with it. I am going to get a .45 1911 and some type of .40 this year hopefully though. Sold my G23 and Colt Defender awhile back.

I've always been 45ACP 1911 and 45 Colt until 2 years ago. I have Degenerative Disk Disease and have lost 2" in height in the last 3 years. This causes me some severe back problems with heavy guns. I bought a Shield 9mm two years ago to carry when my back was giving me a fit when I carried my Colt 45ACP LW Commander. Two weeks ago I bought a Ruger 9mm LW Commander 1911.  I love the 1911 platform. So at this time I use 45ACP, 45 Colt, and 9mm. That's the only 3 calibers that I will have in handguns. I handload so 45ACP and 45 Colt are not an issue. 9mm is cheap enough that I don't handload for it.

Same here... Except my wife got a Colt Agent in .38Special for her BD... So I keep a couple hundred rounds of that on tap too...

10 mm and .40 with the 10mm setup to run .40 as needs dictate
Between these two I'm covered for 75% of my carry needs including out at the farm where problems can be different than in town.

.45 for around the house and 9mm for deep concealment purposes and that one 9mm makes up the other 25% of my carry needs.

The key is accurately shooting what you carry. Caliber 9mm or greater is all that's really important as long as you can shoot it. Accurate hits count way more than caliber of hollow point type

The only caliber I've successfully ditched is .40.  I like my Glocks in 9mm, my 1911s in 45, my revolvers in 38/357 (with the exception of a model 29 that I don't shoot much), and my Rugers in 22lr. I never could fall in love with a gun chambered in .40.

Just 9mm for now, but I do need something smaller for pocket carry, so a .380 is next.

I tried a few years ago... I sold all my .45's and consolidated my autos to 9mm

I found that I really missed shooting .45, so last month I bought my department issue G21SF, and now I'm looking for a Range Officer Operator to fill my 1911 needs again.

With that said, I still had revolver calibers and .380 for the wife

.380
9x19
.45

.38 Special
.44 Magnum

5.56x45
7.62x39
5.45x39

12GA
.22LR

There would be no fun on only having one caliber pistol.

   What we did do though is convert our self defense pistols to one platform.    My wife and I both have Glock 19's for CWP and then we have Glock 20's for our back yard pistols.  I have a big back yard  
Although my wife practices and shoots well it seemed logical for us to go that route. Much like others in rural areas the two legged meth heads are more worrisome that the bears.  Glocks are not my favorite pistol but now with the 42/43 models you get one manual of arms for CC to Woods hiking.   We also have mostly matching AR15's.  
     Even more 3 of our 4 Glocks are gray framed. IE Gray means loaded pull trigger = Bang!!!     Black,FDE etc etc may have a safety may be loaded or not etc.    Her 20sf will get a Gray slide here soon
I've had three times in my life that could have escalated into use of force and I learned that the first two I would have been killed because of stress and lack of being prepared .

Handguns: only 9mm and .22LR for my fun handguns.

For 9mm I use Federal 147gr HST +P for SD

Any good commercial brass for training and plinking works for me.

Guns are tools. Like a mechanic has a box full of tools, it makes no sense to only have one tool. You don't use a hammer to screw in a bolt and you don't use a screwdriver to hammer a nail. A pistol in one caliber does one thing and a rifle as well as a shotgun do other things that the others do not. Even a shade tree mechanic needs more tools than a single hammer or a single screwdriver.

I think there is a middle ground to be had on this subject.

For example. You don't really need 2 rounds that are very close performance wise in the same "form factor" of handgun  (eg... 9 and 40).

It makes perfect sense to have 2-3 auto pistol calibers sized proprortionaly to the gun. Example .380 mouse-subcompact, 9mm compact-full, 10mm full size.  
For defense, if you are going to stick to 1 round. Then stick to one gun type too so mags interchange. Like say GLOCK 17 for HD and GLOCK 26 for CCW.

Also. Rounds vary on availability in a panic. Consider a couple of obscure or east bloc rounds.  During the 08 and 13 panics. 9x18 was still relatively available when 9 and .380 were scarce.  Also, a caliber that isn't associated with high caps (.45, revolver rounds) tends to stay available.

With 4 calibers, keep 2000 rounds for main, 1000 for the rest. That's only 5k and will get you through most panics.

No!  I'm damn near a universal recipient / donor.  .22LR, .32 S&W Long, .32ACP, .380ACP, .38 special,. 357, 9MM, .44mag, .45ACP.  About the only thing I'm missing is .40/10mm/.41.  It makes stocking up for the zombie apocalypse pricey.