The way I figure, is to wrap the chosen yarn around a ruler squishing it together as I go so that each wrap is touching the last with no space in between. I count the number of wraps which fit in one inch and then double that number. This is the number of warps to put on the loom to weave a 1" wide band. Now, having said that, this is not an absolute. The rule works pretty well for me and my weaving style. Others may weave with a little more or less tension than I do. If the width of the piece is important (like it has to fit a particular piece of hardware or something) then I strongly suggest that you weave a test piece first to be certain.
I've also noticed that yarns with a harder twist don't pack together as much as soft and squishy yarns do when weaving, so you might want to take this into account.
Why double it? Because half of the warps will be on the top layer and half will be on the bottom layer at all times.
Here are a few common yarns that I use and the number of warps per inch in my weaving.
You may not get exactly the same result, Everyone's tension and work is different, but it should give you a close idea.
Currently my favorite choice for guitar straps, Omega Sinfonia (you can buy it here: http://creativeyarnsource.com/sinfonia.html) is classified as a "sport weight" yarn and works out to about 40 warps per inch (for me). Similar yarns are Tahki Cotton Classic and Kertzer Super 10.
Sometimes, I use a slightly heavier yarn, like Knit One, Crochet Too's 2nd Time Cotton, listed as a worsted weight and makes about 26 warps per inch (for me).
For smaller things like bracelets and bookmarks, there are the classic #10 crochet cottons, either from Aunt Lydia's or Coats and Clarks. These work out to about 70 warps per inch (for me).
Pearle cottons are also popular choices for inkle weaving, although I think they are softer and don't hold up to heavy use and abrasion as well, so I use them sparingly or on items that are for lighter duty.
A #3 pearle cotton is about 46 warps to the inch and a #5 about 60.