Learn French: Oui, nous en avons une à la campagne. - Yes, we have one in the country.

In French, when you state the quantity of something but don't name the thing itself, you must use the pronoun en (which means "of it" or "of them"). While in English we can just say "we have one," French requires you to literally say "we have one of them" (nous en avons une).

Where exactly does en go in the sentence?

en is a pronoun, and in French, these usually go directly before the conjugated verb. Here, it is placed right before the verb avons: nous en avons (we have of them). The quantity itself (une) stays after the verb.

Why is the word for "one" feminine (une) here?

The word une tells us that the thing they have "one of" is a feminine noun. For example, if they were talking about a house (une maison), you would use une. If they were talking about a garden (un jardin), it would change to nous en avons un.

Why do we use à la for "in the country" instead of dans la?

The phrase à la campagne is a fixed expression that means "in the country" or "in the countryside." French often uses the preposition à for general locations where English uses "in" or "at." You generally only use dans when you want to emphasize being physically inside an enclosed space.