iPhone 13 Pro Max battery health at 84%

Batteries degrade over time with usage due to chemical aging.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%.

iPhone Battery Replacement.

iPhone Battery Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Plan to replace the battery in 4-6 months when the Maximum Capacity is down to 80%.