I have an iPhone 11. Can I use a USB-C 20W power adapter to charge it?

Watt  is like how fast your car can drive, but the speed limit is how fast your car do drive

the iphone draw as much current as it was build to use when charging, the fact that

a charger can provide a lot more just means it's not having its full capacitive

used

when you go to a product

Buy 20W USB-C Power Adapter - Apple

you can scroll down to [Compatibility] and unfold it and view what devices are compatible

If you have a lightning to USB c cable, yes