When planning mashed potatoes for a family dinner, the standard culinary guideline is roughly 1/3 to 1/2 pound (about 5 to 8 ounces, or 150 to 230 grams) of raw potatoes per person.
In practical terms, this breaks down nicely depending on the type of potato you are using:
If you are using medium-sized Russet or Yukon Gold potatoes, one medium potato per person is generally the perfect portion. If your family really loves mashed potatoes or you want to ensure there are leftovers, planning for 1.5 medium potatoes per person is a safe buffer.
If you are using smaller potatoes, such as small red or baby potatoes, aim for about 3 to 4 potatoes per serving.
Keep in mind that once you incorporate butter, milk, or cream, the total volume and richness increase, making 1/2 pound per serving plenty for a satisfying side dish.