How would I make it so a mob gives itself an effect the first time it gets hit then once every couple of hits

Here's an example image. I'm not 100% sure this will work, but I think it will.

This procedure is triggered by "When Entity Hurt" for your mob.

The "duration" of the effect is the number of seconds times 20 (so in this case it's 10 seconds), the level is the potion level (highest drinkable potions obtainable in Survival for Regen are something like 3 or 4, but it goes up to 255). And obviously, you can change what effect it is.

You can change the name of the NBT to whatever you want (nothing other than letters, though, so no spaces or special characters), but make sure all of them are the same.

To change the total number of hits needed, edit the "5" in "Set data number NBT tag 'healTimer' to 5." In this case (obviously) it takes 5 hits.

Again, not sure this will work (I haven't tested it), but I'm like 90% sure it will. Tell me if it does.

I can't edit my reply, but I just realized the procedure will make it take 6 hits, not 5. The number for the number of hits is the total hits needed minus 1.

Also forgot to explain it, this is basically how it works:

Your mob will get, basically, a hit count that starts at 5. Every time it's hit (except for the sixth), that count goes down by 1. When it reaches 0, the next time it's hit, instead of decreasing again, the hit count goes back up to 5 and it gets the effect.