iPhone XR maximum capacity is starting to lower, I have had it only 6 months is that normal?

If you're talking about the battery lifespan (measured by charghing cycles), usually iphones batteries can support 500 charging cycles (on all models):

1 charging cycle: from 0 to 100%

1/2 charging cycle: from 50 to 100%

1/4 charging cycle: from 75 to 100%

and so on.

Keeping in mind that the Battery health value is not accurate (it could be now at 96.99%), I think yours has a normal indication, after 6 months. I charge my device overnight, every night, even if the level is at 80%. The only precaution: do not let your battery go down below 20% level (that's not a good practice with modern Li-Ion batteries).

See also this article:

Batteries - Maximizing Performance - Apple

Regards

Giulio

You're most welcome :-)

Giulio