I used appx. 30 plastic bags for this bag, crochet needle 6mm. The size of needle that works will depend on how wide you cut the strips. I have figured out a way of cutting the plastic bags into one long uninterrupted strip.
Step 1: Step 1 - collect reusable plastic bags
check that all the bags are clean. If they are not wipe them off with a cloth, or maybe even wash them (by hand) and dry them. also check if all bags are in good condition. Some bags may be a bit decomposing, that won't work so nicely anymore, as the strip will then break all the time.