If you use all 16 House Codes and Unit Codes that is 256 addresses.
The EZX10RF can translate all 256 X10 codes and sent them back on the powerline to control X10 modules and devices. No Insteon translation tables used. You can disable individual X10 Addesses if you don’t want thet exact address rebroadcast on the powerline.
I have never tried all 20 addresses of X10 to Insteon so that data I can’t help you with.
The limit of 20 addresses is not really of the addresses, but how many unique ones can be supported. Think of it as if you had a 40 (20-pair) button keypad where you could assign any house/unit to any of the buttons. So, button 1 could be A1, button 2 M2, button 3 B5, etc., until all 20 positions are used.