What is an N20 motor?
This is a manufacturer-specific code that has spread to clones.  You should not expect any great insights from such codes.

Based on these data sheets,

http://www.promoco-motors.com/products/DC/FF-N20%20Series.pdf
http://www.promoco-motors.com/products/DC/FF-N30%20Series.pdf

a reasonable guess is that it is based on the torque rating, with N20 representing 0.2mNm and N30 representing 0.3mNm.

Again, this is manufacturer-specific and not much use in interpreting other manufacturers' part numbers.