Can I use a 20 watt type C charger for my iPhone 7?

A 20 watt charger will be fine, the phone will only draw as much current as it needs. You may have to get a different cable with a lightning connector instead of USB-C.

iPhones 7 wasn’t designed with fast charging, you have to use the 5w charger just make sure that the usb cable and charger are original