I'm sorry for confusing you. The meaning is to prepare 20 to 30 cords. Also, the length of these cords (25cm/10in) is approximate. You don't have to use exact lengths, but I used these lengths. Thank you!
Thanks for asking me! I think there are some differences depending on the manufacturer, but the 3mm used for this (displayed at the time of purchase) was 4mm when I measured it now. Thank you!