How much AC will I need for my 12v DC transformer?
First, a bit of nomenclature. What you have are not transformers. They are DC power supplies. Sorry, but I'm picky about terms.

Second, power supplies like you are buying are usually about 90% efficient, so you'll need about 850/.9, or 950 watts.

Third, your power supplies take 120 volts in, so you will need 120 volt AC power for them. It's true that you can get this from 208 3-phase, but at your level of knowledge that's not a good idea.

So. Will 20 amps at 120 volts do what you want? The power available is 120 x 20, or 2400 watts, more than twice what you need.

Sounds good to me.

Connecting your supplies to AC is best done by connecting a standard power cord to the input of each supply. Start by looking at the data sheet which comes with the supplies. Then take one of your supplies to an electrician, and ask him to show you how to connect it. You should be able to do the others by yourself, but make sure you pay attention to the color coding on the wires. Finally, buy an AC power strip, plug it into the wall socket the show people point you at, then plug the power supplies into the strip. You should be all set, and the power strip will have a master switch which will allow you to turn all your boxes on or off.