1) There is too much water in the bath. 2) How many brothers and sisters do you have? 3) I don't receive many letters nowadays. 4) How much rice do you eat per week? 5) I put too much salt in the soup. 6) How many people were in your house last night? 7) I don't have much time. 8) She didn't ask many questions. 9) My grandfather does not have much hair anymore. 10) How many plates do we need?

Все 10 предложений оформлены правильно: в нужных местах стоит much (для неисчисляемых существительных) и many (для исчисляемых во множественном числе). К каждому — короткий разбор.

1. There is too much water in the bath.
   water — неисчисляемое; с избыточным количеством используем too much.

2. How many brothers and sisters do you have?
   brothers, sisters — исчисляемые; в вопросах о количестве — how many.

3. I don't receive many letters nowadays.
   letters — исчисляемые; в отрицаниях с количеством — many.

4. How much rice do you eat per week?
   rice — неисчисляемое; в вопросах — how much.

5. I put too much salt in the soup.
   salt — неисчисляемое; избыточное количество — too much.

6. How many people were in your house last night?
   people — множественное от person, исчисляемое; вопрос — how many.

7. I don't have much time.
   time — неисчисляемое; в отрицаниях — much.

8. She didn't ask many questions.
   questions — исчисляемые; отрицание — many.

9. My grandfather does not have much hair anymore.
   hair как масса — неисчисляемое; верно much (если речь о волосах вообще, а не “волосках”).

10. How many plates do we need?
    plates — исчисляемые; вопрос — how many.

Небольшая памятка: в утвердительных предложениях разговорный английский чаще предпочитает a lot of/lots of вместо much/many (“I get a lot of letters”), но с too, so, as, how, а также в отрицаниях much/many — норма.