If your going to cross stitch with plastic canvas use 10 count and you'll probably want to use use all six strands. When I do counted cross stitch I usually use 14 count Aida cloth.

I just finished a tissue box cover , I did the cross stitches yarn and only used 2 strands for embeleshing on a 10 count plastic canvas. If you are using embroidery. thead and using the 7 or 8 count canvas, I would think you would need at least 4 strands and possibly all 6. You'd have to be careful that all 6 strands need to lay flat and not twist...could be tricky so the threads don't twist...I've tried it and gave up because it kept twisting and didn't look very ptofessional. Good uck.