99% sure its a #10 in the picture based on the appearance of the zipper chain width and large sized pull. (#10 are 10mm across the chain, same concept for #8, #5, etc.). If you think you're going to load a bunch of weight that will want to spill out and stress the zipper then just go with a #10, keep in mind the zipper pull body is longer and will gobble up more usable length of the zipper if that is of any concern (unlikely on a long pack zipper but a consideration on shorter zippers). My daypack I used a #8 which I'd be surprised to pop but any major hauling will be on the load shelf and not in the pack itself so I wasn't really worried about it.