"Let's not leave too much food."
Translation:음식을 너무 많이 남기지 맙시다.
I think 음식을 많이 남기지 맙시다 is also correct. Please check it and update it. Thanks
많이 just means a lot, but 너무 많이 means too much. That's why 너무 많이 is correct here.
너무 and 많이 are both adverbs, but you can think that 너무 modifies 많이. It's like you don't say 'much too'.