"How many of them did you buy?"
Translation:Combien en avez-vous acheté ?
To understand why your answer was incorrect you need to be aware of the following three points:
Point 1 when elles means them it is acting as a stressed pronoun and stressed pronouns are not generally used in this type of situation.
Stressed pronouns are used in the following situations:
• compound subjects
• after prepositions such as: avec, pour, sans, chez, entre, selon, etc.
• after c’est and ce sont
• after que in comparisons
• when the pronoun has no verb
• with words of emphasis such as: aussi, non plus, seul and surtout
• with même to reinforce the subject
• when the pronoun is the object of the negatives ni … ni … and ne … que
• after verbs that don’t allow preceeding indirect object pronouns such as: avoir affaire à, être à, faire attention à, penser à, etc.
• alone (as in pointing to someone to say him, or raising your hand to get picked)
• after certain verbs such as: avoir besoin de and être amoureux de, and se souvenir (so you do not place the stress pronoun before the verb when the object of these verbs is a person, not a thing).
• in certain imperative statements (dites-moi …)
Point 2 des can either be:
- the plural indefinite article for countable nouns (translated as some)
- contraction of de + les (translated as of the)
Point 3 - Key Point The adverbial pronoun en refers to previously mentioned things or places and can mean:
• some (implying some of it)
• any (implying any of it)
• some of it
• any of it
• of it
• of them
• about it
• about them
• from it
• from them
• by it
• because of it
• from there
Je vais m'en occuper. - I am going to take care of it. / I am going to take care of them
j'en ai autant que toi - I have as much as you
Elle ne veut pas en parler. - she does not want to talk about it
J’adore le chocolat et j’en mange beaucoup. - I love chocolate and I eat a lot of it.
Il fait beau aujourd’hui; profitez-en. - The weather is nice today; enjoy it while you can.
Je ne sais pas combien il y en a - I don't know how many there are of them