Can I safely charge my iphone se 2020 with a 20 watt charger ?

Yes. See: Fast charge your iPhone - Apple Support

and also

About Optimized Battery Charging on your iPhone - Apple Support

According to Apple you should be able to.

However, like some of us, if you’re trying to use a USB-C cable and transformer to fast charge, you may find that the phone stops charging due to what is likely overheating. The original USB to lightning cable will likely work fine though on a 20 watt charger.

yes. you could charge it with a 60 watt charger if you wanted to. the phone will only draw the power it needs.

no problem, glad we could help