every 3 weeks? don't harvest too early you will loose quantity...
If you wish to do every 3,5-4 weeks 12 plants than you will need 24Plants and you will have to put half of your plants on top of something 'couse they should all be at the same distance to the HPS...
It's possible if 24 pots fit in the closet(get square ones!) and you keep the plants small, but maybe you will have to modify your reflector a little bit, but no big deal^^
For shure you can make it less Plants, what ever fits in the closet.
If you really wanna use 400W just to veg them, nice, but better use metal halide then HPS to veg them, the bulb just costs a little bit more, the rest is the same!
To save energy if you are just gonna veg little clones to 25-30cm for a see of green, you can also use a 180W or 200W CFL with a good reflector!
For your clones and mothers under the same light, you need to build something to put your clones on top of it...
It's the same thing like with different size flowering plants, same distance to the light is required.
Anyway if all plants are the same strain anyway, you soon will not need more then 1-2 small mothers to get all the clones you need, couse everytime you cut clones you can cut more the next time from the same mother and this goes really fast after a while.
A friend in Germany, is throwing away lots of clones and gives clones for free to good friends, 'couse he can't use all his clones!
He just has two mothers.(Ed' Rosenthal Superbud and White Skunk)
He uses 7x36W 1,2m flourescents, for rooting his clones, for his mothers, and to veg the little girls.
He uses 2x600W HPS for flowering.
He has usually has about 50 plants flowering, harvesting some every 14 days, at 8weeks flowering...
I'll attach a drawing of my friends equip, you could build almost the same 18/6 area, and use your closet like half of his flowering room.