Q.When to add soil
This year is my first year to grow potatoes in a container (bucket to be precise) and although they are nice and green and sprouting, they are growing at clearly different rates. For instance, one is 8 1/2 inches long, another is about 6 inches long, and the last one is only a mere inch! My concern is that when it is time to add more soil I’ll be covering up the bitty one. What should I do?

You continue to add soil as the plants appear.
Here is a great link with more information.
https://www.gardeningknowhow.com/edible/vegetables/potato/container-potatoes.htm