The numbers below are approximations, there are several factors that affect the amount of CO2 is used up when serving beer. For instance, if you fill a 4L Tallboy Keg at a brewery, depending on how they fill it they will do a good job of preserving the carbonation or they won't and you will have to add it back in.
If you want to serve the whole keg at a dinner party in a single evening, that will be no problem at all. But if you put one liter back in the fridge and leave the regulator set to around 10 psi you’re going to fill a mostly empty get with all your gas. I personally find it better to turn the regulator off and just let the beer go slightly flat. It will still be perfectly fresh, just not as bubbly. There is no sense using up an extra cartridge for that little bit of beer.
Now if you have a 5 or 10 liter keg you might want to hook up to a larger tank. Here are rough estimates of the amount of beer you can serve with each size tank.