It's a common misconception that a kilt has to have a particular number of pleats (and typically someone says something like it has to have 29 pleats, or something like that). Let's look at an example that illustrates very clearly why the tartan tells you how many pleats you'll have.

Suppose you have 8 yards of tartan and an average-size guy (40-42" hips). The kiltmaker will use up about 100" in the apron, underapron, deep pleat, inverted pleat, facings, and center back join. That leaves 188" for the pleats. If you were pleating to the stripe, and the sett were 7", you could make 188/7 or 27 pleats. If the sett were 9", you'd have only 21 pleats. If you were pleating to the sett, it would be a few pleats less than that in each case.