How many watts

No. The phone won't draw more power than it is designed for.

Also see the following from Charge your iPhone with an iPad or Mac notebook power adapter - Apple Support

You can use Apple USB power adapters for iPad and Mac notebooks to charge iPhone, iPod, Apple Watch, AirPods, and other Apple products.

and see: Fast charge your iPhone - Apple Support

No. An iPhone will charge only at 5V or 9V, and will negotiate what to charge at depending on the availability of anything higher than 5V. It starts at a default 5V. A properly designed USB-C Power Delivery power source won't damage anything.

Beside that, the phone itself will have the charging circuits. As long as the voltage is properly regulated by the power adapter, it could theoretically provide infinite current, but the power management circuitry will control the input current.