Everyone has their own opinions on this. Personally I do 20 on the canvas and 60 on the leather before each shave. A good habit to get into is lather up and then do your stropping to give the lather time to do its work and then you will have accomplished good timely stropping and good lathering.

If you look back in time I did a stropping experiment entitled the grand experiment a year or so ago. Interesting reading but my result was 60x on the leather works best.