What kit are you working on? You may need to ask the manufacturer if you are wondering whether or not to use one or two strands for stitching. It should say in the directions for the kit, unless you are missing a page (which the manufacturer could replace, but not ECS, as they are a retailer and don't have spare parts for the kits).
Most floss comes in 6 strand pieces that you can separate, but some kits, like Riolis use wool, and others use off brands of floss. But it's impossible to answer not knowing the details of the kit.