Theoretically your 5G keg will consume 150g of CO2 by the time beer is done. Plus 60g if you use CO2 to purge the filled keg prior to kegging (as you should). So 2kg should last 9-10 kegs assuming that's all you use it for and don't overcarb beer or purge multiple times. And have zero leaks etc.
Realistically I think 5-8 is close to real life experiences.
Keep in mind also that 2kg tank won't hold full 2kg unless it's very cold.