Note: Great mashed potatoes hate recipes. Although the amounts here 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.