can I use power adapter 20w on my iphone 11 or should i use 5w as recommended, i'm just want to be sure and dont want to degrade my battery faster.

You can use the 20W (or larger including a 96W Ma power adapter).

See the following from Fast charge your iPhone - Apple Support

Fast charging works when you use an Apple USB-C to Lightning cable and one of these adapters:

• Apple 18W, 20W,2 29W, 30W, 61W, 87W, or 96W USB-C Power Adapter

I'm not sure where you saw that a 5W was recommended. The 20W is the main one currently sold for iPhones.

Only pre-iPhone 11 models came with the 5W USB power adapters. Regardless, yes, you can use the Apple 20W power adapter with your iPhone without any negative effects to its battery.

Ref:

• About Apple USB power adapters - Apple Support
• About the battery and performance of iPhone 11 and later - Apple Support

i wouldn’t say “recommend” …

… Apple after all has spent a considerable sum on both battery and charge controller development so as to ensure that their millions of devices are safe, usable, AND reliable …

… but I would say that if you don’t NEED to use fast charging, there’s certainly no compelling reason to do so.

20w adaptor will work great on your iPhone 11 and enable fast charging.

You likely will notice the phone get warmer.

This extra heat does “stress” the battery but it’s designed for the added stress.

The actual charge controller is inside the phone, so it will always maintain the charge rate within the as-designed constraints.

From an engineering perspective less stress is generally better.

“How much better” always makes for a lively discussion which in which I’ll not engage here.

Certainly no harm will ever come from using an older 5W power adaptor.

While I personally have a full range of “18W to 97W” power adaptors; the bulk of my iPhone Xs Max charging is at night with a trusty 5W USB-A “icecube.” (it probably came in the w/ my iPhone 5)

However, on a “heavy use / power-sapping” day and when in need of a charge; I NEVER hesitate to use one of the more powerful adaptors.