"What is the population?"
Translation:Πόσο είναι ο πληθυσμός;
I was wondering the same thing. Troll says elsewhere that Πόσο always asks for the quantity. https://forum.duolingo.com/comment/18099210 (For those wondering what we are talking about: πόσος is nominative masculine singular, so we thought it might work here in agreement with ο πληθυσμός, while πόσο is neuter nominative singular, neuter accusative singular or masculine accusative singular.)