To show the amount of something / quantity
A. HOW MANY:
How many + countable noun (plural)
How many languages do you speak?
Note: How many language do you speak? FALSE
How much + uncoutable noun (singular)
How much water do you drink every day?
Note: How much waters do you drink every day? FALSE
To show the price of some goods or services.
–How much is this notebook ?
–How much does this book cost ?