I am thinking you are talking about an ebb and flow set up and if so and growing from seed, I would start 6 or 7 and hope for about 4 females to grow out and this is with about a 4 to 5 week veg, but believe me it will be a tight fit, but if you are able to get a good set of clones from the first run I would put 6 to 8 in there and veg for 2 weeks.
Adieu