How long's a piece floss? Sorry. Seriously I think it really depends on the fabric. I prefer to stitch on 28 count and 2 strands gives a lovely coverage if stitching over two - however if stitching over one then a single strand gives a lovely delicate appearance to your work.
I've just about finished a small scissor keeper/key fob designed by Sue Hawkins and this uses six strands - now that is a really interesting effect.
So a short answer this week. Must go stitch..............