First you'll need to work out how many kg of potatoes you eat typically per week.
Now multiple by 52 to see how many kg of potatoes you'll need for the year.
Each potato plant (grown from a single tuber) typically gives a yield of between 4 and 7 kg. So to be conservative, divide the total potatoes needed for the year by 4 to give you the total number of tubers/seed potatoes you'll need.
6 seed potato tubers weigh about 0.5 kg, so then you can work out how many kg you will need to order.
A further consideration is the harvest timing and the storage of the potatoes. By storage we mean the ability of the potato to be stored, how long you can leave it stored in the ground, and how much pest-free storage space your family has access to. This influences how many 1st earlies, 2 earlies and main crop seed potatoes to buy. This is because 1st & 2nd early seed potatoes do not store as well, but start feeding your family earlier in the year. Maincrop potatoes store better, and can be left in the ground longer, but are ready to harvest later.
And finally you need to consider how much growing space you have/how much of that space you want to give to potato plants! You need about one square foot per plant, and also need to consider rotating your plot each year for three years.
Here's an example
Family eats 2kg of potatoes per week
This x 52 weeks = 104kg of potatoes eaten each year by the family.
104kg needed /4kg harvest per potato plant = 26 potato plants needed = 26 seed potato tubers needed ( or maybe 30-35 tubers if you assume you might lose some plants and want to play it safe)
26 tubers /6 tuber per kg= rough 4.5 kg of seed potatoes needed to feed the family.
Now figure out what percentage of this 4.5 kg should be 1st early seed potatoes versus 2nd early seed potatoes vs. main crop seed potatoes.