Preheat oven 350. Drizzle garlic with oil. Cover tightly with foil and bake 30-40. Let cool until easy to handle (can be done a day before). Squeeze out garlic.
Cover potatoes with water (peeled and chopped--however many potatoes you'd like) and bring to a boil. Cook 20-30 mins until fork tender. Drain, reserving water. Mash potatoes with margerine and garlic, adding enough potato water back in to bring it to desired consistency. Add salt and pepper to taste.
I always use 5 pounds to potatoes, and double the garlic. I also add chives just before serving. It's great if you add a sweet potato or two into the potatoes--adds a nice color!
I use the leftover gravy (made from seasoned thickened veg stock), plus leftoever potato water, the peels from the potatoes and the skins from the garlic as the base for my next pot of soup stock.