A single primary regulator will work for an unlimited number of kegs at once, provided that all kegs are carbonated to the same volumes of CO2, and all set to the same serving pressure. A primary regulator is one that attaches to the CO2 tank, and often includes an additional gauge to monitor the tank's level as well.
If you want multiple serving/carbing pressures at one time, you'll need to buy a secondary regulator (or a bank of them, one for each line). Note that this is not usually necessary. I've been running 3 kegs for some time now with only a primary regulator. I just used some stainless T's to split the gas line coming off the regulator.
Here's a picture of what I'm talking about. The regulator on the far right is a primary, and would attach to the CO2 tank. The ones to its left are all secondaries. The primary in this case could either go straight to your kegs (if all the same pressure), or it could go into this bank of secondaries, and each one of them control a different line.
