Does anyone know what it means when the Objective says: "4/12 Worker's house at 7?"

I can only assume it means there are not enough workers, but what do the numbers mean?

I've attached a screenshot.

I guess, that means you need 12 worker houses (= normal houses) at level 7, you have just 4.

But when I build more houses the number doesn't seem to change. What else do I have to do to get Level 7?

Leftclick on a house and you should see what futher criteria you have to meet.