I use two chicken breasts, the smallest bag of red potatoes and skip the red pepper flakes. Recipe commenters recommend:

• When you add the chicken drop it down to 395 and cook for 22-23 minutes
• Use same amount of garlic sauce for half the chicken amt
• Add 1 TB brown sugar  in addition to 2 in recipe and use 2 tsp red pepper flakes

A Food Network Recipe

Ingredients

• 2 pounds small red-skinned potatoes, quartered
• 3 tablespoons extra-virgin olive oil
• 1/2 teaspoon cumin seeds (optional)
• Kosher salt and freshly ground pepper
• 4 cloves garlic, finely chopped
• 2 tablespoons packed light brown sugar
• 1 lemon (1/2 juiced, 1/2 cut into wedges)
• Pinch of red pepper flakes
• 4 skinless, boneless chicken breasts (1 1/2 to 1 3/4 pounds)
• 2 tablespoons chopped fresh cilantro or parsley

Directions

Position a rack in the lower third of the oven and preheat to 425 degrees F. Toss the potatoes with 1 tablespoon olive oil, the cumin seeds, 3/4 teaspoon salt, and pepper to taste. Spread in a large baking dish and roast until the potatoes begin to brown, 25 to 30 minutes.

Meanwhile, heat the remaining 2 tablespoons olive oil in a small skillet over medium heat. Add the garlic and cook, stirring frequently, until lightly golden, about 2 minutes. Remove from the heat and stir in the brown sugar, lemon juice and red pepper flakes.

Remove the baking dish from the oven, push the potatoes to the sides and arrange the chicken breasts in the middle. Season the chicken with salt and drizzle with the garlic mixture. Return to the oven and bake until the chicken is cooked through and the potatoes are tender, about 20 minutes. Remove from the oven; transfer the chicken to a cutting board and slice. Add the cilantro to the baking dish and toss with the potatoes. Serve the chicken with the potatoes and lemon wedges. Drizzle with the pan juices.

http://www.foodnetwork.com/recipes/food-network-kitchens/garlic-chicken-and-potatoes-recipe.html