To figure out your number of ends you multiply the sett by the width. If you have a 10-dent reed and you want your scarf to be 8’’ wide you would have 80 warp ends. But I’m not sure if that is your question exactly. Is it that you want to know how to sett your yarn? IAre you on a rigid-heddle loom? It is a way gross overgeneralization, but roughly speaking, bulky works in a 5-dent, worsted/DK in an 8-dent, sport in a 10-dent, and fingering in a 12-dent. Of course it depends on the type of yarn and how much it will bloom. This is a post I wrote about a new sett checking tool, but the top part gives you an idea of how to guesstimate sett with just a ruler.