English has two types of nouns, countable nouns (also called "count nouns") and mass nouns (also called "non-count" or "uncountable").
(Many mass nouns can also be used as countable nouns in certain cases.)
Countable nouns take the determiners "many" and "few".
Mass nouns take the deerminers "much" and "(a) little".
"A lot of" works with both types. So does "some".
Countable nouns can be singular or plural - for example, "boy", "girl", "man", "woman", "computer", "book", "horse".
Mass nouns are always grammatically singular - for example, "mud", "dirt", "honour", "grace", "dignity", "joy". They don't form plurals.
With a countable noun, a question such as "How many men are present?" could lead to an answer such as "many", "a lot", "a few", "not many".
With a mass noun, a question such as "How much aluminium will we need?" could lead to an answer such as "a lot" or "a little" or "not much". ("Much" is a less likely response because "much" is a polarity word that occurs primarily in either negative or interrogative phrases - though there are some idiomatic exceptions.)
The form of the question also tells you the possible responses. If the question asks "How many" then it must be asking about a countable noun, and if it asks "How much" then it must be asking about a mass noun.
Countable nouns can be counted - so "how many" can lead to a response such as "one" or "four".
Mass nouns can only be measured, so "how much" can lead to a response such as "ten metres", "ten litres", "a handful" or "a spoonful".