There are two important factors with HP ammo for defense use...
Does it perform well as a defensive round...
Does it function 100% in the pistol it's to be used in...
The first question can be answered on a basic level by looking at gel tests. So long as they are done properly. One with several shots is nice, as that gives a good average. Having several different people with tests is better, as lot numbers will be different usually. So if they all are consistent, you can reasonably assume the ammo will perform similarly for you.
Hornady critical defense has been shown to perform well. I can not remember the projectile size, but it is at least 124gr or more.
A note: Lighter than 124gr projectiles tend to be less consistent in their performance. So avoid those rounds... Some manufacturers offer several weights in their HP ammo, so know what you are picking up.
The second question can only be determined by test firing the ammo in the pistol you wish to use.
How much testing you need to be comfortable and confident in its performance in your pistol is up to you.
Some may say one 20rd box is enough for them... Cost may be a factor there... Others say a hundred rounds... Or two hundred.
Me, cost is a factor, but I need more than one box. I use at least 50 rounds as a minimum. More if I can, a hundred and I am pretty confident, but I do periodically test a mag or two as well, to ensure it remains viable ammo for me.