Basically they have a huge battery in them, 160 KWH / I think you can get up to 300 KWh batteries in them and they use those to augment your charge. So lets say it has 25 or 50 coming from the grid, that'd be a 240 volt 100amp / 200 amp single phase feed.
Your car can take 75 kwatt rate to charge, it'll take what it can from the grid, the rest it gets out of it's battery to charge your car up at what it can handle. Once you are done, the grid keeps going to recharge it's internal battery back up to 100 percent again.
The main selling point of this is, it's a LOT easier to run your 'mains' out to one of these, even if it's at a fairly low KW rate, than to put in a new 300kva transformer at several thousands of dollars to power a regular charger. It's not perfect but does the best it can in your situation.
To answer your question, yes you could probably do 3 or 4 nearly empty cars before it'd need to probably do 6 hours or so to fully charge back up again. Just because it CAN charge at a 200 kw rate, YOU could say to only charge at a 100 kw rate, then it's taking the 50 from mains, and only 50 from battery, that'd give you 6 cars worth of charge before battery totally dead. it could be made to work pretty well.
If your demand is that high that it'd constantly be ran dry then you might want to consider putting a regular station in
aaron