All good advise above from experienced allotmenteers, not sure how my advise would translate onto an alloment but hope it does. Have tried main crop in veg beds in the back garden
I grow in bags mainly and try to keep some spuds back in March/Arpil to plant in May to prolong the harvest. Spuds are happy to chit for a few months if you keep them in the right conditions. I've bought spuds in January from spud fairs and started chitting right away but not planted some out till May.
A lot of growers rate Charlotte but I don't, never had a good crop and found they don't cook well. Grown several different varities, there's heaps to try but this year kept some in their bags after the foilage died back. Dug up a 2nd early - wilja last week and the spuds were fine. I've still two bags of spring grown spuds to harvest and don't buy spuds from about late June through to December.
The problem with growing Christmas spuds is when you come to harvest in December and this is why they can't be grown on an allotment, you can't dig them up, the ground is frozen. Last yr mine were put in an unheated GH and the bags froze, they were fine in February when the bags had defrosted.
Spuds will only grow for the length of time nature intended, so spuds planted in July for Christmas -1st and 2nd earlies - the foilage will have died back well before December and I wouldn't waste my space in the GH for Christmas spuds any more but would put them in a place which is frost free like garage or basement.
In answer to your last Q, how many spuds do you need to plant, depends on how many spuds you eat