i would say a 5 pounder would be great. you could use paintball tanks but unless you have a few already it would be easier and cheaper to get a 5. I have a 5 pounder running 3 kegs now and I have carbonated and served 7 kegs so far but I know its about to run out.
I run 2 five gallon cornies on a 5# tank. I'm on my fourth keg and should be able to push the rest of these 2 out before I need to refill. I also use it to carb as well as push. If you naturally carb you would probably be able to push 10-15.
I use a 10 lb tank for 3 taps, carbonating and serving. If you can handle the size, 10 lbs is great! Here's the best place to buy any size CO2 tanks... get the reconditioned ones since most places just trade out anyway:
i'm another on who runs a two tapper off of a 5# tank. i can get around 5 or 6 kegs carbed and served off the 5 pounder, i keep two tanks on hand in case of emergency.
get whatever you can find for cheep! I have (2) 40lb tanks and am going on 2 years on the current one. The cost for larger tanks isn't much a step up. I'd get as big as you have space for. Now of course the larger they are, the more expensive leaks are. So check your system!
Thanks for all of the information everybody! I just finished building my kegerator two days ago and was curious how long my 5# tank would work running two taps.