Here's the completed dialogue with "How much" or "How many":

Mai: (1) **How many** tomatoes and onions do you need for the sauce, Lan?
Lan: Three tomatoes and two onions.
Mai: OK. Do you need any flour?
Lan: Yes, we do. (2) **How much** flour have we got?
Mai: Half a kilo. (3) **How many** eggs do you need?
Lan: Three. And (4) **How much** milk do we have?
Mai: Only 500 ml.
Lan: 500 ml is probably enough. Is there any cooking oil left?
Mai: Yes, there is. (5) **How much** oil do you need? We've only got 100 ml.
Lan: That's fine.