The basic rule as I understand it is 50 watts per plant/5k lumens per plant. 20 plants under a 1000watt hid...50 watts per plant, 5k lumens per plant, perfect. You can go vertical, use light movers etc (or both!) and go 40+ plants but I wouldn't fuck with something like that unless you already got the more straightforward shit people are doing, down pat. If you are still mastering the variables involved in "the basic stationary, horizontal light" grow and you try some experimentation and it goes to shit, you won't know if it was one of the basic variables that went wrong or if it was something about the experimental method its self. Good luck!
ps. assuming your 1000watter is hps why don't you use an mh conversion bulb for veg??