Good question! It'll be a little hit and miss to start with, but you'll get a better idea of the variables year by year
- Yield per plant - depends on cultivar, weather, soil, pests and diseases, cultivation method etc.
- Storage conditions
- Eating rate - your family may eat more home grown spuds because they're so good!
We grow for just the two of us, earlies/second earlies only (a new lottie neighbour always grows more maincrop than he needs and gives us a free sack that sees us through winter).
In the past, for a family of four eating potatoes 2-3 times a week, I planned for seed of 3kg 1st earlies, 3kg 2nd earlies and 9 kg mains is about right. At least it's a reasonable starting point!
Chit or not to chit - whatever you prefer, only don't chit maincrops.