NOTE: Great mashed potatoes hate recipes. Although the above amounts will make a fine mash, you're better off with these simple formulas. Try to hit a 2 to 1 ratio of russets to reds (by weight), and have at least 1/4 cup of dairy per pound of potatoes. You may not use it all, but it's good to know it's there. As for the garlic, use your best judgment.
Combine potatoes in a large pot and just cover with hot tap water. Place over high heat and season water with 1 to 2 teaspoons of salt, (it should taste like sea water). Cover the pot and bring to a boil.
Meanwhile, combine dairy and garlic in a small saucepan and bring to a simmer over medium heat. Keep this mixture barely simmering until the potatoes are done.
As soon as the water comes to a boil, remove the lid and reduce the heat to a simmer. Cook for 15 to 20 minutes or until a potato chunk can easily be crushed with a pair of tongs. Drain, then return potatoes to the pan, return pot to heat and shake for 30 seconds so the surface water can evaporate. Remove pot and set on a towel or hot pads. Pour about half the garlic mixture into the potatoes and mash with an old-fashioned potato masher. Start tasting and looking at the consistency right away. If mashers seem dry or bland, add more of the garlic mixture. Avoid over mashing or you'll end up with gluey instead of fluffy
Serve straight or garnish with any of the following: parsley, chopped scallions, crumbled bacon, sun dried tomatoes, (If you have the dry ones, make sure you re-hydrate them.) grated horseradish, horseradish sauce, pesto, more bacon, Sauteed mushrooms...use your imagination.
(Recipe courtesy Alton Brown)