Is it safe to use 96W adapter and what is the recommended watts to maintain battery health

Yes, you may use the 96W adapter to charge your iPhone. The iPhone’s operating system will control the required amount of watts/current to charge the battery automatically.

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Battery Health & Performance 

Your battery health is fine and is undergoing normal usage. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. You should change out the battery once your Battery Health reaches below 80% which will take over 2 years to reach under normal usage. Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➡️ iPhone Battery and Performance - Apple Support

Axel F.  

The more wattage generate more heat which is not good for the battery. I would not recommend a 96W adapter to charge your battery. The 20W charger is just fine. You would not get the full 96W to the phone as it will automatically reduce the wattage to a lower level, but it will still generate unecessary heat.