I think everyone should have a 9mm of some sort. I have absolutely no rational response as to why or anything to base my comment on, I just do. But as others have said, it can get out of hand and keeping a good stock of ammo in each caliber can get expensive. I firearm for every application that you'll be shooting is what I like to live by. But that's what got me into the position I'm in with calibers.
.22, .223, .45, .380, 9mm, 32-20 and I'm not nearly as diverse as some.
But yes, you need a 9mm.