Is there any problem with using the 12 W charger instead of the given 5 W charger? Does it effect the battery health? Or will it cause any battery issues in future?
iPhone XS Max, iOS 12
No. 10w, 12w, 18w, etc.. chargers are perfectly compatible with all iOS devices.
It will not cause any battery issues.
A 12 W USB power source will charge the phone faster (about 1.5 times faster). A USB-C power source and USB-C to Lightning cable will charge it about twice as fast. None of these will hurt the phone or the battery; it was designed to accommodate any power source that can plug into the phone. The most convenient is a 7.5 watt Qi wireless charger. It won't be as fast, but you can just leave the phone on it when you're not using it. Here is my treatise on the subject-->When to charge your iPhone or iPad - Apple Community
12 W charger.
