Odd question. You don't had routers (or any device) for the sake of adding them.
When designing a network, you first get your requirements.
You usually take into account:
- current needs
- expected evolution in the next few years and scalability
- security (intrusion, malicious usage, etc...)
- implementation time
- and of course... ...cost (including maintenance cost)
(additional factors may be relevant as well)
There's always many, or at least several possible solutions using various technologies. It is the work of a network architect to design the solution.
The solution chosen will dictate the number of devices.
All networks evolve, so you extend them and add devices when there's a need to.
In medium to large setup, you will have a number of routers larger that what is strictly necessary to have things running, to provide redundancy. So if a router is down, either due to a bug, a hardware failure or a planned maintenance, the network is not impacted.