how many cars are there. why we are not use how Mach

Because "how much" is used for uncountable nouns and "how many" is used for countable nouns and car is a countable noun.So, we'll use here "how many cars are there?".

there is a basic difference between many and much. * if it's countable we will use " many" ex;- how many cars? * if it's not countable we will use "much" ex:- how much milk?

many use for always count. much use for always quantity.