Learn Portuguese (Portugal): Quanta água querem eles beber?

The word for "how much" or "how many" changes to match the noun it describes. Because água (water) is a feminine singular noun, quanto changes to its feminine singular form, quanta. If you were asking about a masculine noun like leite (milk), you would use Quanto leite.

Why is eles sandwiched between querem and beber?

In Portuguese questions, it is very common to put the subject (eles) immediately after the conjugated verb (querem). You will often see this reversed word order in questions, though you might also hear Quanta água eles querem beber? in casual everyday speech.

Could I leave eles out completely?

Yes, absolutely! Because the verb ending of querem already tells us the subject is "they" (or the formal "you all"), Portuguese speakers frequently drop the pronoun. You will often hear this simply as Quanta água querem beber?

Why doesn't beber change its ending to match eles?

When two verbs work together like this (wanting to do something), only the first verb is conjugated. Querem is conjugated to match "they", while the second verb, beber, stays in its raw dictionary form (the infinitive). This is exactly like English, where we say "want to drink" rather than "want to drinks".