Your maris piper already have shoots on them. This is what the term chitting means. Some people dont chit potatoes, most do, it just gives them a good start before you put them in the ground. If the shoots are two inchs long they are ready to go in the ground. As for the amount you get off each plant. A guess would be about between ten and fifteen from an early potato prob less from a main. If you are planing on feeding a family, think you should go for more than ten seed spuds, my guess is about thirty if you have the space. When they have grown cut off all the green growth above the soil to help keep blight at bay. Or lift all the spuds and store in paper sacks in a frost free dark, cool shed.