I have bought 20w apple fast charger for iPhone X does it charges fast or may harms my iPhone X

Your phone is specifically designed to use that power adaptor.

You may notice that the phone gets warmer than “earlier phones” while charging … but unlike earlier phones it’s designed that way.

For max battery life you should always enable “Optimize” in your device’s battery settings and try to follow a “routine” charging schedule to maximize the benefits of “Optimize.”

Read more here:

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