With a five pound bottle, I think I was able to carb and serve around five kegs. I don't remember exactly. If you could swing it, I would go with a 20lb bottle. It's normally cheaper per pound with a bigger bottle. In NC I was getting a 5lb'er for about $18, but my 20lb'er cost about $30-$35.
The general rule if you're force carbing is 1-2 kegs per lb of CO2. Twice that if you're naturally carbing. That's assuming no leaks.
I would say get the biggest tank that you can fit in your fridge/keezer (if that's where it's going to be located). If it's not going in the fridge, get the biggest one you can. You'll fill it up much less often and as nitrousjunkie said it's much cheaper ($/lb) to fill the bigger tanks.
Buy as big as you can afford and have room for (I'd go 20lb) then keep your eyes open on Craigs List and try to pick up a spare 5lb tank on the cheap. It's nice to have two tanks for the inevitable "Oh crap! I'm out of CO2!" in the middle of a party. You can also carb off line and use CO2 to run cleaner etc through your kegs if you pick up another regulator...
If you are patient you can find stuff pretty cheap on CL or at garage sales.
I vote 2 5's over a 10 as well. I would also look into where you plan on having your refills done. The beverage center closest to me will only do their exchange program with 5 gallon tanks. I like the 10 gallon tank I have much better than the 5 (it seems to last a lot longer). A 2LB paintball gun tank can also make a nice backup.