‘Much’ and ‘many’ are both determiners that suggest an unspecified quantity, with more or less the same basic meaning: ‘in great quantity’ or ‘in large number’. There is, however, a distinction in their usage.
‘Much’, not ‘many’, is used for uncountable nouns, which are in singular form.
I don't have much faith in him.
Here, 'faith' is an uncountable noun and hence, we use 'much' instead of 'many'.
How much money do you have in your wallet?
In this example, 'money' is an uncountable noun and hence, we use 'much'. Note that 'dollars' or 'rupees' are countable as we say 'ten dollars' or 'ten rupees'.
There was much compassion in his voice.
Here, compassion is an uncountable noun and so we use 'much' and not 'many'
On the other hand, ‘many’ is used for countable nouns, which are in plural form, and here ‘much’ cannot be used.
Many youngsters today are taking to atheism.
'Youngster' is a countable noun and hence, we use 'many'.
How many days remain?
In this example, 'Day' is a countable noun.
There are many obstructions ahead for us.
Here, 'Obstruction' is a countable noun and this means that we have to use 'many' and not 'much'.