At what battery capacity is it recommended to have my battery replaced on a iPhone 11 Pro Max?

I don't know that there is an exact number the indicates you NEED to replace it.

The battery for sure should be fine when 80% and higher.

Once in the 70s, it should still work fine.. probably start to notice that it doesn't last as long as it used to.

Here is a support article on iPhone batteries. Maybe just wait until the Battery Health tells you that your battery is degraded. or if you experience unexpected shut downs.. or any of the symptoms listed in the article.

iPhone Battery and Performance - Apple Support

Replace it when the battery health is 80% or less.

As per apple official recommendation, it is always advisable if battery percentage goes below 80% down from its original performance.