First, make sure the beer is done, unless you intend to have the carbonation uncertain. Check into fast ferment on these threads. Generally, about 3-4 ozs in a keg (with a CO2 infusion as a "cap") will work, but it depends on how far your beer has attenuated and how many volumes you want for the style of beer. With CO2 available, why not just keg and carb with CO2?