Cross-stitch's entire supply question is two multiplications and a habit. From the learn page's floss section, worked out loud:
Design stitches ÷ fabric count = design inches. A 140-stitch-wide chart on 14-count Aida is 10 inches — 14 is the stitches-per-inch, the count's whole meaning. Then add the border: 3 inches is the standard, every side, for the cut. Ten inches of design wants a 16-inch fabric width, and that's the number the shop needs from you.
One DMC skein carries 8 m of six strands; cross-stitch usually works two of them on 14-count. As actually stitched, a skein carries about 1,700 two-strand stitches on 14-count — so a 100×100 two-color design runs roughly a dozen skein-uses between its two colors. Working three strands on 11-count, or one on 18 for the fine stuff? Same skein, thinner splits of it — the counts change, the arithmetic doesn't.
Two skeins of each color and thank yourself later — the arithmetic has a remainder, the project has a mid-way crisis, and a dye-lot mismatch is the one mess even honest charts can't fix. The app computes skeins per color straight from the chart and tells you exactly what each number is counting.
Dangle the needle, count the wraps, stitch something this week. — the bench crew