"What ply is this yarn?" I get asked this a lot - and it is actually harder to answer than you would think. "Ply" is mainly used in the UK, Australia and New Zealand to describe the thickness of a yarn or the so-called "yarn weight".
"Ply" made sense when yarn was made from wool: The higher the ply (=the number of strands plied together), the thicker the yarn.
With the wide variety of yarns and fibres on the market now, "ply" is no longer that useful. To add to the confusion, there are many names for yarn weight across the world: What is "8ply" in Australia is "DK" or "double knit" in the UK and "light" or sometimes "light worsted" in the US.
European yarn labels will generally provide you with a recommended needle size and tension but usually do not specify a particular yarn weight or "ply". To give you an example: For a classic 8ply (DK) yarn the recommended needle size would be 4mm, with a tension of 22sts per 10cm.
There have been attempts to standardise yarn weights internationally. One such organisation is the American Craft Yarn Council which has developed standards and guidelines for crochet and knitting.
The Craft Yarn Council does not give any indication of "ply". If you want to understand how "ply" relates to the international yarn weights, you can use the classification used in the yarn and pattern library Ravelry as a guide.
Irrespective of what "ply" or yarn weight your yarn is, you should always check the recommended tension and needle size and work a tension square to make sure that it matches your pattern. Adjust your needle size if necessary.
Doing a tension square is tedious and it is easy to think you can just skip it. Don't. Believe me, it is far more annoying having to redo a whole project because your tension is off and you didn't do that tension square before you started.