"How many biscuits?"

"Many" is used with countable nouns. As you can count biscuits 1, 2, 3... you have to use "many".

"Much" is used for uncountable nouns.

"How much flour and sugar do I need for making biscuits and how many eggs?"

Flour and sugar are uncountable, eggs are countable.

"How much money have you got? How many bills and how many coins?"

Money as a whole is uncountable, but separate bills and coins are countable.

Look here for more info: <a href="https://dictionary.cambridge.org/grammar/british-grammar/much-many-a-lot-of-lots-of-quantifiers" rel="noopener noreferrer" target="_blank">https://dictionary.cambridge.org/grammar/british-grammar/much-many-a-lot-of-lots-of-quantifiers</a>;

Not many biscuits.

Why? Because I'm on a diet.