Learn Italian: Quanti euro hai? - How many euros do you have?

In Italian, the word euro is invariable, meaning it doesn't change its ending in the plural. Whether you have un euro (one euro) or novanta euro (ninety euros), the word stays exactly the same!

Why is it Quanti and not Quanto?

The word for "how much" or "how many" changes its ending to match the noun it describes. Because we are asking about multiple euros, and euro is a masculine word, quanto becomes the masculine plural quanti.

How would I answer this question using a number like 45?

You would simply say Ho quarantacinque euro. You use ho (I have), followed by the number, and then the currency. Notice how we don't need to say io (I) because ho already tells us who is holding the money.

Where are the words for "do" and "you" in the Italian sentence?

Italian doesn't use helper verbs like "do" to form questions—you just change your tone of voice to make it a question! Also, the "you" is already built into the verb hai (you have), so the pronoun tu is usually left out.