Well, you haven’t got a mesh at all at the moment. 
How many routers you need will depend entirely on the layout and structure of your home. Zigbee signals are very weak (by design - this is what allows you to have small battery-powered sensors); they don’t go far, they don’t pass through walls well and they’re easily swamped by interference.
It’s not a question of linking A to B - that’s wi-fi thinking - there have to be many different possible pathways so that the routers can select the best one for each message.
You need to start by building a mains-powered network that covers all 12 rooms. When that’s solid, you can think about adding battery-powered sensor devices. I would be thinking in terms of 40-50 mains-powered routing devices.
There’s no ideal ratio between devices and routers. The best scenario is one where every light and eveny wall socket in the building is a router, with a few additional dedicated devices like your Sonoff. If it’s any use as an example, my house has small rooms and a thick wall across the middle which used to be an exterior wall. Getting past that old wall into the newer part of the building is quite tricky. I have three or four routers in every room (one room has nine) and more on the stairs between floors.
Lots of good advice here: