Hi Betty,
I would suggest that you try all 6 strands of floss on a corner of your 6-count fabric, do 4-6 stitches in a row and see how good the coverage is on the fabric. Too thick? Remove a strand and stitch 4-6 stitches again to check what YOU are satisfied with. Doesn’t give the coverage you want? add one strand and ‘test’ Stitch again.
I personally do the above anytime I begin a new pattern because I followed the written instructions to use 2 strands of floss on a pattern and was getting too much ‘see-thru’ of the fabric. Keep in mind that the more strands of floss you use, the more floss you will need for the project.
Some stitchers prefer fabric ‘see-thru’, I don’t.
So I do self-determination as how many strands of floss to use on the start of a new project. 🙂