Use one bag of large marshmallows(10 oz.) per six cups of cereal. You can use 7 oz. marshmallow creme or 4 cups miniature marshmallows instead.
Here is the original 1939 recipe from Kellogs
Ingredients 3 tablespoons butter or margarine 1 package (10 oz., about 40) regular marshmallows - OR - 4 cups miniature marshmallows 6 cups Rice Krispies®
Stove-Top Directions 1. In large saucepan melt butter over low heat. Add marshmallows and stir until completely melted. Remove from heat. 2. Add KELLOGG'S RICE KRISPIES cereal. Stir until well coated. 3. Using buttered spatula or wax paper evenly press mixture into 13 x 9 x 2-inch pan coated with cooking spray. Cool. Cut into 2-inch squares. Best if served the same day.
MICROWAVE DIRECTIONS: In microwave-safe bowl heat butter and marshmallows on HIGH for 3 minutes, stirring after 2 minutes. Stir until smooth. Follow steps 2 and 3 above. Microwave cooking times may vary.