Many is used with countable nouns (things you can count, like employees, books or cars).
Example: How many employees work in your company? (You can count the number of employees.)

Much is used with uncountable nouns (things you can’t count individually, like water, money or information).
Example: How much water is in the bottle? (You can’t count water in separate units.)

It’s a simple rule, but it makes a big difference!