I did plant all of the seed potatoes on the same day - I don't normally bother with earthing up, I just fill the containers when I plant them (unless I run out of compost). I was just surprised that twice the number of potatoes produced very little extra crop.Here is the blog trial I mentioned

Plant Pot Size Guide - Jacksons Nurseries Plant Pot Size Guide - Jacksons Nurseries

For example, a dwarf variety can grow in a 6 inch pot but most beefsteak varieties will need a much larger area for their roots.As valid, and long winded, as these points may be, they don’t get to the heart of the matter. Better growing methods mean you can use smaller pots

Pots - the Ultimate Guide by Fantastic How to Grow Potatoes in Pots - the Ultimate Guide by Fantastic

So now that the potatoes have been planted that’s it, right? Wrong. There is more work to be done during the growing process. This additional work is called hilling. I did come across an article last year which tried a few different things inc different number of potatoes in each bag and using small vs large seed potatoes etc. It is particularly important for those feeding organically that there is plenty of oxygen in the root system because soil microbes need oxygen too! The humble potato has been a staple of the British diet since the end of the 16th century, it has earned its place in the pantheon of British dishes by adding tantalising delicacies such as mashed potatoes and roast potatoes, along with many others, to our palette.I'm a bit gutted I planted only 2 penellype I wanted to try the 4 potato thing in a 30l pot but forgot! Last year with Quadgrows my yields were the best yet with 5lb of Zenith salad tomatoes from a plant with 4 trusses. They also developed very rapidly, saying Quadgrow is great if length of season is a concern. It should come as no surprise then that, potatoes are one of the UK’s most farmed crops and have become a favourite among those who grow their own food. Normally potatoes require a lot of space to grow. When planted in the ground, it is recommended to leave at least 45 cm between each row. Cover the potatoes with approximately 5-10 cm of soil, don’t worry if this doesn’t seem like enough you will understand why it should be done like this later.