A double knitting thickness knitting yarn is around 6ply - that is 6 oz per 560yard skein
I believe that 20/2nM is just finer then a 2 ply, so just less than 1/3rd the thickness of the double knitting. You would divide the larger number by the smaller to get the finished weight of the yarn.

Double knitting is usually about 3metres per gram, shown as 6/2
English 4 ply is 4.5 metres per gram a 9/2
3 ply is 6 metres per gram, a 12/2
2ply is 9, a 18/2
1ply is 18, 36/2

A common weight of industrial yarn is 13.5 nM and it is between 1 and 2 ply.

I have been using these count systems and converting between them for a long time so combined with a sliderule and a bit of approximation I can translate fairly freely..

Anne the Pleater