Good question. Here are some of the reasons we are not as specific about the number of potatoes to use.

In foodservice

No. 1 Idaho potatoes are packed in a 50 lb carton and the sizes (which chefs often specify) vary by approximately how many potatoes fit into a box. So, a 50 count is about one pound for each potato.

No. 2 potatoes sold in foodservice can vary too, some chefs order a 10 ounce and above size, especially if doing French fries.

In retail

The produce manager may stock loose potatoes which they order from the same foodservice cartons. The loose ones are often purchased by consumers that want a baked potato, so they are bigger than what you find in a 5 lb bag.

A retailer often orders a 70 or 80 count potato for their displays. The bags available at the grocery store can contain multiple sizes, from 2 ½ ounces and up.

The closest answer

USDA specs are based on a 5.3-ounce potato, about 100 calories. Based on that, you'd need about 3 potatoes to make a pound. 

Hope that helps!