English makes a disctinction not made by any other language known by the author. In English there is a difference between asking for single elements and asking for an amount.
amount: How much water is needed to take a bath?
single elements: How many cars are parked on the parking space.
Quanto / quanta corresponds to how much and quantos / quantas to how many.
Both, quanto / quanta and quantos / quantas can be used as interrogative adjectives and interrogative pronouns.
How many hairs do people have on their head?
How many will come?
Used as a pronoun obviously anyone taking part in the conversation has to know to what the pronoun refers to.
Both quanto / quanta and quantos / quantas has to agree in gender and number with the noun they refer to.
how many adjective => asks for the amount of single elements
Quantos minutos você ainda irá demorar para chegar?
In how many minutes you will be here?
Quantos passageiros desembarcaram?
How many passagers went out?
Quantos alunos chegaram atrasados hoje?
How many students came too late?
Quantos dias tenho de ficar aqui?
How many days I must stay here?
how many pronoun => asks for the amount of elementes of a known group
How many are missing?
Quantas foram atingidas?
How many were hit?
Quantas foram multadas no mês passado?
How many received a penalty bill last month?
how much adjective => asks for the amount of a known homogenous mass
Quanto dinheiro foi roubado?*
How much money has been stolen?
Quanta água do mar já bebeste?
How much seawater did you drink?
* Remember what has been said in 11.2. In opposite to all other roman languages the pretérito perfeito simples can be used as well to describe an action that has an impact on the present of the speaker. We can therefore translate it with the present perfect.