I always buy a bit extra than what I think we will need - if there are leftovers, so be it, but if there is not enough it is annoying and a hassle to have to go to the shops to get more. If we are having a lot of people over we tend to buy a carton or two of cans of soft drink, then we will have maybe a big drink bottle filled with punch or cordial or even just water as a back up. Extra milk is handy too, for cuppas.