Can I charge iPad using iPhone 12 charger (20w)?

The iPad Air 2 came with a 10W USB power adapter. The iPhone 12 did not come with any adapters. However, if you are referring to the Apple 20W USB power adapter; yes, it can be used with this iPad with the appropriate charging cable.

Ref: About Apple USB power adapters - Apple Support

Which iPad model do you have?

Axel F.