du = de+le, so your question becomes when to use/not to use the article with the word "de". This was first briefly discussed in the tips and notes of the food chapter: https://www.duolingo.com/skill/fr/Food . In this sentence, we have the word "combien", which expresses a quantity. The article is omitted after quantity words (combien de, un peu de, beaucoup de, une centaine de, trop de, etc.). This particular case of quantity words is explained at least in: https://www.duolingo.com/skill/fr/Adverbs-1 .