Pizza is tricky. If you make one at home, it will be according to your recipe (bread makers are typically the healthier option as you control all your ingredients and you can go low on the salt and perservatives.)
Frozen crusts, pre-made crusts, and the roll out types of crust that are packaged, usually about 6 to 8 servings per container, just as Tirzah Laughs mentioned. If you go out to eat, there could be as many as two or three servings in just once slice! Pizza hut for example, has one slice of its largest pizza that measures 3 1/2 inches at the crust. This same slice can contain more than 3 times your daily sodium and saturated fat allowances.
If you are watching your weight or using a diet system such as weight watchers, you'll have to look up each restaurant before you order and you an typically find out what they consider one serving and the nutritional information.
Hope this helps.