While 9mm and .22 long rifle will be the “most plentiful”, reliance upon securing your resources from a rapidly dwindling supply would be folly.
Lay in several bricks of .22 long rifle and many 50 rd. boxes of 9mm. Of course this assumes you have the firearms to digest such loads.
But remember one thing, as has been said before, a pistol is only for shooting your way to your rifle.
And in a "SHTF" situation, there will be plenty of both laying all over the ground.
I have long and short weapons in the same chambering, learned that from the cowboys!!!
A pistol is actually most useful for dispatching the wounded after the fighting dies down....
The weapons doing the most killing will be shotguns and .30 caliber rifles.