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

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)

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)