I do not focus on a single caliber. However, when reloading, I tend to focus on some calibers more. I like the versatility of the 44 magnum.
For my self defence carry guns, I like 9mm, 40 S&W, 45 ACP, and 10mm. Note than anything, I carry 9mm. The 10mm has a lot of flexibility. If premium ammo is not available, I will feel best served by the 45 ACP.
I have handguns in a few calibers that I bought or inherited over the years and it helps now that some calibers are tougher to find.
For my ammo stockpiles, I get what is available. I do not let inventory run low.