I have a relative who has accumulated somewhere in the neighborhood of 5000rds because he swears by the .22 caliber. My opinion is that it's not enough to put someone down unless you hit them between the eyes.
The key to .22 ammo for SHTF isn't necessarily for your own defense. You can still have your .45 or .308 or 12ga or whatever and have enough ammo for it.
When I talk about .22 ammo it's to stockpile as trading stock and use both. Almost every closet in America has a .22 rifle or pistol. The ammo is cheap, easy to store, lightwieght, easy to divide into smaller amounts and will work for killing things to eat and defense as well.