Plies don't mean much in today's world. You can have a fingering weight or bulky weight that are 3 ply so it's best to tell us what weight the yarn is and gauge.
That said..everybody knits at different gauges. The best way to get the size you want is by doing a swatch and then measuring to see if its what you want. Cast on 20 or 25 depending on the gauge of the yarn and knit in the stitch you want to use.