It's tough to single out a single food and determine what is too much. You need to look at your total calorie load and macronutrient distribution. If you are very physically active and have eaten very lean throughout the day, you could eat a lot of pizza. If you have high calorie foods, alcohol, and deserts, your wiggle room is low. The type of pizza matters too. Deep dish meat lovers is much more calorically dense then a thin crust veggie pizza.