I persoanally would do 3, 10 pound bags. That allots a pound of mashed potatoes per person. Not that everyone will eat a pound, but some will go back for seconds, and you may have a small amount for leftovers. I like to have too much rather than too little, so I err on the side of caution and make more.
I add milk a little at a time, a little sour cream, some salt, and butter. I also take whole garlic heads and wrap them in foil and bake them for about an hour, when you take them out, you can squeeze the cloves right into the bowlm and mash the soft garlic right in with the potatoes...it's amazing.
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