Towel Curls

Some users' comments on Towel Curls (from here):
 * I did my first towel curls yesterday -- felt like a complete idiot. Wondering if the towel was too short, or my legs are too long, or.. or.. aaagh. :D
 * The trick is to have a really big towel - do not support the curl with your foot - really use it as weight - if you're confident then apply some pressure - really gets my biceps pumping
 * I never felt right doing these either, attached weight to the towel and did them that way instead.
 * Comparing my kettlebell curls to towel curls... I'd say that I'm doing the equivalent of 50 lb curls with the towel.

How many should I do? (ref)
 * The book's description of the exercise says to just do 5 towel curls, as it's all you need if you are doing it properly. But towel curls are the second exercise of some supersets, which means you have to do a set of 12. How did other people end up doing this?
 * Actually it's 6-12 for the second exercise of a superset, and 6 of these seems to be plenty.