What battery percentage should I charge my iPhone 12 at?

If you have optimized charging activated (it is active by default), then your iPhone will learn your charging pattern. For example, if you usually charge your iPhone at night, your iPhone will charge directly to 80% and keep your iPhone there - until it knows you're going to get up soon, then it will charge the last 20% up to 100%

This puts less wear and tear on the battery. But it takes some time before it learns the charging pattern, and if you have different patterns, it is not certain that the function will activate.

But you can charge your iPhone to 100%, and make sure that when it is at 100%, disconnect it. Also, try to avoid draining your battery completely.

You can find more useful information about your iPhone Battery here:

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple