Much употребляется с неисчисляемыми существительными, а many - с исчисляемыми.
1. How much rice is there? - Not much.
2. How many potatoes are there? - Not many.
3. How many biscuits are there? - Not many.
4. How much milk is there? - Not much.
5. How many apples are there? - Not many.