Can i use portronics 20W adpater to charge my iPhone 11. Is that safe or cause any problems l…?

Yes, it does support fast charge so there will be no problems. Sure, it is going to get hot, but it's not the phone's problems, it really depends by the charging speed. I suggest that you use the original cable included, you most likely shouldn't trust low-quality chargers (both cables and bricks)

You can use it. See the following from Fast charge your iPhone - Apple Support

(Bold added by me)

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
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)