Can I use 20w adapter to charge my iphone 11 pro max?

Yes. All Apple chargers and cables are cross-compatible, as long as the connectors fit without forcing.

Yes, but it won’t charge any faster that the xxW 18W charger will.

Actually, a 20w charger will charge faster than a 13w charger.

Fast charge your iPhone - Apple Support

Perhaps you meant the included 18w charger. Then I would agree with you.

My mistake, or rather typo; I meant 18W.

I believe the 20 watt charger will charge an iPhone 12 faster than the 18 W, however.

Lawrence Finch wrote:

I believe the 20 watt charger will charge an iPhone 12 faster than the 18 W, however.

Yes, indeed that’s what I observed with my iPhone 12 Pro.