Can I use a 20 watt charger for my iPhone 11?

Yes it is safe assuming it is an Apple charger and it will not damage your battery. There are other Apple certified chargers as well. This article may help:

https://support.apple.com/en-us/HT208137

As elcpu stated, any apple charger - up to their largest for a MBP … will work well ans safely w/ you iPhone 11

Your new 20W charger … paired w/ a USB-C to Lightning cable … will allow the phone to “fast charge” as-described in the link posted.

sadman183 wrote:

Yes it’s an Apple original charger.

However, I’m worried because apple recommends 5 watt charger for iPhone 11.

No worries, I do it all the time and have never had an issue. The internals of the iPhone will prevent any "overcharge".

Yes … that’s perfectly normal.