To take them one by one:
Hamburger is normally used as a countable noun (a hamburger, 2 hamburgers). You should normally talk how many hamburgers were eaten, using the plural.
Fish & chips is syntactically plural, but is often treated as a non-count noun. So you can say "Fish and chips (is/are) popular in the uk." And write about how much fish and chips was eaten. You can also use a counter: "How many servings of fish and chips..."
Pizza as a countable noun refers to a whole pie. It can also be used as a non-count noun, referring to a type of food. You can use both "How much was spent on (pizza/pizzas) ..." but in this use I'd prefer the non-count, singular.
There aren't many rules for this. The countability of nouns can't always be guessed, however, a quick glance at dictionary definitions of hamburger and pizza show the current usage for countability and plural formation.