Is 87% battery capacity good for an iPhone 11 Pro Max?

87% would be expected if the phone was 15-18 months old. If your phone is 24 months or 2 years old, you are ahead of the game.

Replace that battery when the Capacity gets down to 80%.

With luck, you might be able to go another 8-10 months before the battery needs to be replaced.

Lithium ion batteries degrade with usage, mainly the number of charge/discharge cycles. The expected (minimum) service life for an Apple phone battery is usually 500 cycles (most models) and Apple considers a battery which has degraded to 80% will probably need replacement. iPhone 11 family is a few years old now, so 87% is probably within the expected range and it is still above the 80% threshold.

It’s OK. Some do better or worse than the nominal rating. Not much you can do about it.