"You are registered to the flat number"
I don't think so. The Czech population registry works with addresses, but does not contain the exact apartment you live in. In case of blocks of flats, there might be ~ 100 apartments with the same address.
It is different if you are the owner. The land registry does contain finer ownership data up to the level of the apartment.
But as a renting person, while your contract will contain the apartment number, the authorities won't be registering that. You may actually go completely officially unregistered for years if you never visit the town hall and ask for a registration. It is not a hard requirement to get registered as a renter.
But post will be delivered to you nonetheless if your post box is labeled with your name and someone knows your address.