Much and many are actually two words that are very similar in the English language, but they have few differences between them. The first basic difference is that; "many" is usually used with countable nouns, while "much" is usually used with uncountable nouns. Examples of these uncountable nouns of which you can use "much" with include water, money, sleep, time, pain, and etc. On the other hand, examples of countable nouns of which you can use "many" with include: days, pen, clothe, human being, shoes, etc.
Using it in a sentence, for example, you can say, "how much time did you spend there?" On another note, you can say, "how many days did you spend there"? "Many" is most likely used with plural nouns, while "much" is usually used with singular nouns. In affirmative sentences, "many" can be explicitly used, but you must be careful to rightly use "much" together with the right words to make grammatical sense.