The number of strands of floss to use is, as with most of counted cross stitch, open to individual choice. Traditionally, a certain amount of the background cloth should be visible. However, some people prefer a full, covered look. Some common choices are two or three strands for 14 stitches per inch, two strands for 18 stitches per inch, and three or four strands for 11 stitches per inch. Try a few stitches on a scrap of the project's fabric to see if the look is what you want.
source : www.mismatch.co.uk