The number of potatoes you use will solely depend upon how many people you are feeding. I have seen this done in a 9x13 dish and in an 8x8 for a smaller serving.
Dry Packet of French Onion or Onion Soup
1/4 Olive Oil
Rinse potatoes and boil for 15-20 minutes. While boiling, prepare the "topping". Mix 1/4 cup olive oil and 1 packet of dried french onion soup in a gallon-size ziploc bag. Cool potatoes for a few minutes and cut into bite-size pieces. Put potatoes into the bag, seal the bag and mix into the seasoning.
Bake in greased dish on 350-375 (depending on your oven) for about an hour or until the potatoes are cooked through.