Depends on the type string material you are using and your preferences.
16 strands of B-55 or other polyester material would be way overbuilt IMO. You could probably get away with 10, but that's on the low end IMO.
With Dynaflight '97, 10-12 with the loops padded to 18.
With 8125, 12 with the loops padded to 18.
With BCY-X or 8190, I'd go with 18 strands with the loops padded to 24.
You can go up in serving size for a proper nock fit, or build up your nock point with dental floss. Padding under the serving is doable, but I personally don't care for it.