768 square feet, assuming you need to fit your fowering, cloning, and vegging as well as mothers all in this space, I would do the following (keep in mind this is just me, there will be a million different answers and none are "correct").
First thing I would do is section off a space that is 10x12, roughly 120 square feet. I would put two 1,000watt MH light in here giving you right at 3,000 lumens per square foot. This would be good for the mothers and vegetating plants. If you cycle plants through the flowering room in batches, you would have enough space in here for two large mothers, and enough room for vegging plants (meanining you would run 3 cycles, therefore harvesting roughly 1/3 of your garden every 20 days assuming a 60 day strain). You also need to put a small shelf in this space for a cloning station. Ventilation would be too much to get into here. However, if you want to veg your plants to a height of more than one foot, you will probably need more light.
For the flowering area you would have roughly 648 feet left over, that is a LOT of space and you STILL have over 10.8 square feet of space PER PLANT after devoting 120 square feet to the veg/mother/clone area. How you choose to use that is completely up to you, it really depends on how you want to grow. If the purpose of the grow is to get as much weight as possibe then you can fit 60 monsters in there easy. If it was me I would cut the space down because the amount of lights needed to get enough lumens in a space that large would be a big number. 648 square feet times 5,000 lumens per square foot =3,240,000 lumens. This menas you need 22 1,000 watt HPS lights putting out 145,000 lumens each to give you the MINIMUM amount of light needed for that space....a lot of coin. I would cut the space down a lot...but that is just me.