In an 8- to 10-quart pan over high heat, bring 1 1/2 quarts water to a boil. Peel potatoes and cut into 1-inch chunks. Add potatoes and garlic to boiling water; cover and return to a boil, then reduce heat to medium and simmer until potatoes mash easily, 12 to 15 minutes. Drain and pour into the bowl of a heavy-duty electric mixer fitted with a wire whisk, or return to pan.
Add butter and 1/2 cup cream. Beat with electric mixer on medium speed, or by hand with a heavy-duty whisk, just until smooth. If mixture is stiffer than you like, beat in more cream until potatoes reach desired consistency. Add salt and pepper to taste. Scrape into a serving bowl.