Because the canvas or linen component is more aggressive to the razor's edge generally less is better, at least compared to leather. I usually do 40 linen/60 leather (never less than 100 laps total), but like Obie I'll up the ratio for harder blades or stiffer grinds.

It's a good sign that your razor needs a touchup when it stops responding to linen stropping, no matter how many laps.