Like jdoiv, I use my CO2 bottle for more than just serving beer, so it is hard to say, by my experience, how many kegs you will get. Just make sure you know where your local gas dealer is, so that you can refill quickly when you need to, and it won't matter.
I like to use the CO2 to push start a syphon when racking to a secondary or to the keg, etc. Also, like jdoiv does, I use it to move sanitizer around, either between kegs, or syphoning from a carboy, etc. On top of that, it also gets used if I feel like hauling out the counter-pressure bottle filler. If you ever decide to get a filtering system, you will use some of your CO2 on that also, as you push the beer through the filter, from one keg to another.
A full CO2 bottle is a wonderful thing to have around.
I definitely have to agree with Chris though, once you get started, it is like everything else in home brewing, it spreads faster than bacterial grown in an unwashed carboy.