Is that sentence grammatically correct?

Hm. The words are in the right order, but it is not complete. What is it you want to say?

It is only a sentence fragment and, as such, makes no sense. It can be used as a noun, a direct object or an indirect object.

Is the focus of the question on the word “amount”? . Pizaa can be counted either by slice or the whole pizza. So a person who has no idea how many were eaten ask “What was the amount of pizza eaten….?” If they measure it by weight “the amount of pizza eaten was…a lot, or 10 kg…” .I’m guessing.

It doesn’t have a full stop.

“The amount of pizza eaten….” what? What about it?