Necesitábamos tres sillas más.

English Translation

We needed three more chairs.

Can someone help me understand why the imperfect is used here instead of the preterit? Without further context, it seems like a completed past action due to the sentence ending without anything else interrupting the action.

I think it’s because it’s not an action but a state. Consider “No teníamos suficientes sillas”, which has a similar meaning. Or “Nos faltaban tres sillas”, for that matter.