combien
how much (price, quantity)
how many (used alone in spoken questions)
Ça coûte combien ?
How much does it cost?
how many (used with de + noun to ask about number/amount)
Vous êtes combien ?
How many of you are there?
which/what day
On est le combien ?
What day is it?
Notes
GENERAL INFO
Combien means "how much" or "how many." It's an interrogative adverb, often functioning as an interrogative pronoun in questions to ask about quantity.
USAGE
Use Combien de + noun to ask about quantity: "Combien de livres as-tu ?" (How many books do you have?).
It can also be used alone as a pronoun: "Combien coûte cela ?" (How much does that cost?).
TRAPS
Always use "de" (or "d'") after combien when followed by a noun, regardless of the noun's gender or number.
Never use "des," "du," or "de la."
Example: "Combien de pommes?" (How many apples?).
