when you scrog a plant, you are intentionally causing it to cover a larger horizontal plane
IMHO 2x2 is a good space for an untrained plant vegged for 3 weeks
i say that to say that my answer would be 1
1 plant - about 4 weeks of healthy veg
with only 1, i'd go for about 2/3 coverage and prolly do 2 screens about 6 or 8 inches apart
if you've got HID lighting, i'd just put 2 side by side and cut the lights at 15 inchesish and bend the top twice
1 time at the switch and 1 time at the second stack
if you don't care about plant count, i'd get the 1/2 gallon grow bags plant 24 seeds
go under 3 or 4 24 watt cfls for about 18 or 20 days, intentionally keeping them dainty
day 18 or 20, switch and crank up your big lights for flower
so either 1, 2, or 24 IMO