plant numbers have nothing to do with it, its about your veg height to when you flower ratio...

so if you have time to veg longer you could have less plants, and it depends on the strain your growing, some are better for SOG growing while other grow big and bushy.

a good grower w/o co2 will get around .5 grams per watt in hydro, not saying hydro yields more, just takes more practice to get it perfect in soil. So you have 800 watts total so at .5 gpw that would be 400 grams = 14 ounces.

so basically what you are doing seems to be a sea of green, 24 plants under 2 400's is alot unles you flower them very short..

its all strain dependent, and what growing style you use, like if you top or lst it first, but say you had 24 plants and yielded arond half ounce per plant, that would be 12 ounces...but say you had 12 plants, vegged them a little longer you could get an ounce or over per plant and would still equal around 12.

whatever type of growing method you plan to use, get the strain accordingly that will be suit that. and remember its better to have too many plants and having to kill some because of overcrowdization, than to not have enough and regret you didnt plant more.

You should get anywhere from 10-16 ounces saying you have an ideal growing environment and grow the strain well.