Square density is the count of stitches per unit of area, usually expressed in Stitches/SqYd or Stitches/m2
Each stitch carries one yarn, if the yarn is a monofilament it will have several tapes (filaments) per stitch. In other words, a stitch is the hole in the backing where the yarns pass through
Depending on the turf design one stitch may have more than one yarn.
When using Square Density as a measure of the quality of a product, you should also consider the face weight (total weight), and how many filaments cross through each stitch, and how heavy these filaments are (linear density).