"Il ristorante usa una gran quantità di pane ogni giorno."
Traduzione:The restaurant uses a large quantity of bread every day.
A 'quantity' is a certain amount of something, like "a bucket full" or "100 grams". You can have 'many quantities' (for example, all the different quantities of ingredients you put into a cake) but if you're trying to say that there is a lot of any one of those ingredients (like,700 eggs) then it's a 'large quantity' not 'many quantity'.
Many eggs = a large quantity of eggs. Many quantities = lots of different amounts. Many quantity = (wrong).