20w charger is suitable for iphone 11 pro max if not then tell me it effects battery health or not.?

You can safely use the 20W charger with your iPhone 11. The charging is controlled by the phone, not by the charger, and the phone will only draw what it needs.

See this Apple tech note→About recalibration of battery health reporting in iOS 14.5 - Apple Support

But there’s no reason not to use your phone while it’s charging. The best practice is to charge the phone overnight, every night, and only charge during the day if it goes below 20%.

READ the link I posted.