How do you determine singles WPI based on how many plies you want? well, that is in part a function of twist angle or grist, so all I can give you are my ball parks. I tend to spin woolen, for an airy yarn, but will spin high-twist sock yarns. More on twist angle (and yards per pound) in a little bit.
For a two-ply, I estimate my singles at 3/2 the WPI of the result; so for a result of 14 WPI 2-ply (thin end of CYCA 3, which includes DK) I'd want to spin my singles 21 WPI.
For a three-ply, I estimate my singles at twice the WPI of the result; so for a result of 14 WPI 3-ply, I'd spin the singles at 28 WPI.
Mabel Ross (Essentials of Yarn Design for Handspinners) and Anne Fields (Spinning Wool: Beyond the Basics) both give tables in their books that are empirical numbers; if you work out ratios, they are in the neighborhood of my estimates above, though there is variation from WPI to WPI and TPI (twist angle/twists per inch) to TPI -- soo, to continue with all these silly letters, YMMV (grin)
Twist angle/twists per inch has a big impact on yards per pound -- so if you are copying a commercial yarn, this can be a pretty big factor in getting similar YPP for their WPI.
Yards per pound is a function of wraps per inch, twist angle and spinning style: if you spin a lofty, airy yarn that holds alot of air (woolen style), it will be lighter than the same thickness of yarn spun by compressing all the air out of it as you spin (worsted style). So there are huge variations in what the YPP will be for a given WPI; Aldon Amos described it well in his tome, The Big Book of Handspinning. I'm not sure what other references there are for it though I'm sure Mabel Ross (my fave!) touches on it.
I put together a table on CYCA/WPI/YPP a while back in discussing how much fiber you need to get 200 yards of yarn. Here's the data:
Weight WPI YPP
0-lace 24+ 2600++
1-fingering 19-22 2600+
2-sport 15-18 1900-2600
3-DK 12-14 900-1800
4-worsted 9-11 500-1100
5-bulky 7-8 300-500
6-super bulky 6 or less 400 or less
As you can see, those YPP ranges are _really_wide_
Also for those who check details -- there is no CYCA 0, but then again, I guess they don't believe in laceweight yarn? So I made it up since we all love spinning the stuff!
Also also, the plied yarn as spun still isn't "done" -- many yarns will bloom when you wash them so their final post-wash WPI is lower (i.e., the yarn thickens) than their as-spun WPI.
(based on a reply by me on Spin-List)
The picture at the top is a 3-ply superwash merino spun to fingering weight for socks. I plan on churning it in the 72 needle cylinder of my sock machine next-but-one (there's another skein of handspun in line in front of it, for socks for a dear friend). The spinning of that yarn was discussed in Do you really measure twists per inch? ... and you can find several interesting posts by doing a blog search on twist angle. Enjoy!
If you have a rule of thumb for singles' WPI versus 2, 3 or more plies, please add it in the comments here.