What should I say? “Wie viel Tomaten” or „Wie viele Tomaten”?
You will get following answers to your questions: "Wie viel ...?" "Ein Pfund, bitte." -- "Wie viele ...?" "Fünf Stück, bitte." You see in general you should use "wie viel" for non countable things, like sugar, water and so on. If the things are countable you may better ask for the number, like "wie viele". It's exactly the same difference like between how much and how many.