"Beware the man who has only one gun, he probably knows how to use it."
The above adage from Thunder Ranch President Clint Smith is but one of his many quotable quotes. In this day and age where many shooters have an AR for every purpose, it is an important quote to consider. Now I don't believe he was saying nobody should own just one firearm, far from it. I believe the point and purpose of the saying is a reminder that it is better to master one discipline or one gun, than it is to be pretty decent with a few of them. Instead of buying some more parts or accessories for a gun that isn't your primary, you're better served buying more ammo to train with (or professional training if you haven't had any).
Take for example, my old Gen 3 Glock 17. I have put more rounds downrange with that than I have with any other (civilian world) gun. I know what kind of ammo it likes and what it doesn't. It is the one I'm fastest with above all others that I own. Manipulation drills are all second nature. I shot my first deer with it, a running head shot at 25 yards on a Sitka Blacktail.
This isn't to say I'm an amazing shot or a master pistoleer, this is to point out that when you maximize a strength, you can put yourself into a position to leverage that strength. ONE gun in your collection should be your primary, go-to firearm. It should be the one you spend the most time doing dry-fire drills with, one you take to the range every time you go, and preferably is the one you carry with you every day. Find the ammo it likes best, then feed it regularly.
Master the one gun.