Like many food items, it can be either countable or uncountable. I ate two pizzas. How many pizzas do we have? That's a nice pizza. In these sentences, we are using it as a countable noun. A pizza is an object, and we can count them. But if you say: Can I have a piece of pizza? Do you want some pizza? This is too big for me. I'll share it with you. How much pizza do want? One slice or two? then it's uncountable. We are imagining a large pizza, which is cut up into smaller portions. Here it's the same as saying 'Can I have a bowl of soup?'