I'd get a 5# or larger. I like a 10# cylinder- it fits in the fridge ok, and lasts longer than the smaller tank (unless you have a leak!).
You can do this several ways- use a WYE fitting for a couple of kegs, or a distributor for the gas for many kegs. I have both, actually. I have a 4-way distributor on one side of the WYE and just a quick disconnect for the other WYE. That's mostly because I want to top off a carboy with a shot of co2, or fill a soda bottle and use a carbonator cap, or to push the sanitizer through without taking anything off of the distributor. Like everything I do in brewing, it may be overkill for many!
You can even simpler T fittings, but make sure you have check valves installed. More pressure in one keg and cause backflow into your regulator or other tanks if you don't add a check valve.
One regulator will work for multiple kegs if they are all at the same PSI. If you want different pressures, you'll need a regulator for each setting. If you want to do soda on one, and beer on another, for example, you'd need two primary regulators. Still one co2 thank, though!