In the last post I talked about the important relationship between warp and weft sizes when weaving tapestry. These questions are intimately connected to sett.
Sett is most easily understood as the number of warp threads in a unit of measure. In the USA we use inches, in Europe you probably use centimeters. (Don't get me started on why the stubborn United States never switched to metric. Yes, Europe and Canada, your way is better.)
So think about your loom. If you hold a ruler up to your warp and count the number of warp ends that occur in one inch, you will know your sett. Like this.