I prefer to work with two strands of thread so I can start my sewing off with the loop method. I have stitched with three strands on 14 count aida (on the family of frogs project I did years ago), this produces a real bumpy and raised effect I'm not sure I like. I don't really like stitching with one thread because it doesn't offer enough coverage, in fact I think I'm only comfortable doing one thread on really fine counts, 18 count and above.
I hate it when you get cover kits that give you 16 count aida and then tell you to only use one strand. Quite a few of the DMC freebie kits are like that. I'm stitching another Tom Mouse Country Companions design at work during my lunch hours and this uses only one thread over 16. I don't like it. I usually ignore the instructions and stitch those kits with two strands anyway, and if I run out of thread I can usually just raid my DMC collection. That's what I did with the Canterbury Bell Fairy that I finished earlier this year.