There's just 2 of us as well. Last year I had about 24 potato plants (mostly charlotte as I find them quite good small as well as letting them grow on). And that was way too many, gave away a lot, even though it was a dry year and I was away for a month so they didn't get watered for the whole of August so the yield was probably quite low.
You could plant some 1st early as well (Charlotte is 2nd early)l, for an earlier start to the crop, and you could plant them in batches, so that the later planted once mature a bit later (you'll have some smaller spuds later on, rather than them all getting big).
(1st earlies mature in 70-90 days. 2nd earlies in 90-110 days and maincrops in 110-145 days. They can all be planted on the same day - or Maincrop first if the plot for them is ready first
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Personally I wouldn't grow maincrop unless you have space. They are not easy to store - depends on variety, but many will be sprouting soon after Christmas unless you have some chilled storage - farmers have excellent chilled storage facilities and, except in bad harvest years, a sack of spuds from the farm gate is cheap
We grow Maincrop because we don't like any chemicals being put on our food ... but I would grow every other vegetable we eat before giving space to maincrop spuds; I have plenty of space though.
An other problem with Maincrop is that because they are in the ground much longer they live on into the Blight season, so there is a much higher chance of getting diseased than an Early variety, which will be out of the ground by the time Blight becomes rife, in an average year.