Biggest Room In Danchi Rules

My mother is the household of our danchi. We live in the biggest room that required 5 people to be qualified. If I leave, should they leave the danchi too? Because there is no more 5 people living there

Is this a regular danchi? Or is it something like a kodan or kasetsu-juutaku? Each establishment has a different policy. Ask your landlord or agent or whoever is responsible.