Using a quick one:
Divide watts by voltage to get amps. If you can't find any usable numbers, take the wattage of your bulbs on a per fixture basis and use that to make your calculations.
Keep in mind 1800 watts is a 15-amp circuit and 2400 watts is a 20amp circuit. Make sure you do not exceed the amperage of the circuit, and if possible, front end your power with your own power distro, even something like a Furman rack mount unit. Always better to pop your breaker in that because you can reset it at your convenience. But this also involves you knowing what else is using any circuits. If you're sharing a circuit and not know it, you could pop it before you exceed the current draw you needed without even knowing it, and then you pop house breakers.
Popping house breakers is not a big deal, they are there for protection purposes. It's just good courtesy to not have to go into their wall panel whenever possible. Unless you're using pigtails and pulling right off the rails, in which case I hope you're a licensed electrician. Otherwise, have someone videotape your tie-in and send it to one of those funniest home video shows. Just keep tape running when the meat wagon arrives, OK?
24 Par64's. Wow, that's a lot of current. I've seen bigger, trust me. How about 2 40-foot trusses with 40 each for a total of 80? Of course, they aren't all on at once either.
Using 300 watts per fixture:
300WattsX24Fixture=7200Watts
7200-watts/120-volts=60 amps. So, Jingle's numbers are right on for that. That's 2 20-amp circuits at 100% or 4 15-amp circuits at 100%. that's a LOT of juice coming out from the wall.
If Jingle's makes an argument for LED's, listen to it. Then you could probably run the amount of fixtures same on a single circuit and get same output with less heat. but you'd probably not need as many thanks to the color mixing capabilities.
Now assuming the DP-415, and 15-amp total capacity, you're looking at running 8 300-watt fixtures off this? I don't think so. That's 2400 watts, and that means you're gonna pop that device's breaker, running full on. And hence, we've already shown you have 45-amp total capacity on dimmers/switcher packs, assuming 100% on utilization. For your rig, at 100%, you draw 60 amps, so you would need 1 more DP-415, no argument, and run 6 off each dimmer pack, so you need to figure out where you want to double up. 6X300=1800 watts. Works.
Not how I'd do it, but then again I organize a tad more logically. But doing it how I described above would be acceptable, safe and by the book and should result in no problems.
Check your bulbs, because if you're pulling 500 watts per fixture: Thats 12000 watts/120 volts=100 amps, and you're gonna need a buttload more DP-415's!! You'd need 1 per each 3 Par cans and you'd technically lose 300-watts per unit(so to speak) because you'd not really be able to use the full 5 amps per channel, because you're maxed out at 15 amps per logical unit.
Hopefully it's 300-watt bulbs.