@new8n 

@Virgo is right. If you use 2.4GHz radio, you may set up 3 radios on channels 1, 6, and 11 to avoid interference, but for six radios, there will be a lot of interference. One option is switching to 5GHz CPE.

Installed openwrt,

planning to use

802.11r wifi fast transition

  @new8n 

If you mean, you want to overcome the interference with fast roaming, these are completely two different concepts. Access Point is not like a torch that projects signal in one direction, the antenna leaks signal all around the access point but with different power, obviously the most powerful in the front. When two APs are in the same channel, the physical layer prevents them from sending signals at the same time, so random back-off algorithms come into place. AP only send a signal if the channel is clear, but it's not that straightforward in reality, so a lot of resources would be wasted because they sense noise on the channel. You only have three options, either 1)buy 5GHz radio, 2) change the topology and don't install them back to back (you may consider installing at different heights on the same pole), 3) or accept the interference and live with it which could simply fails you.

If I use square topology instead keeping routers at center:

if

1 is main broadcasting router

2 gets internet from 1

3 gets internet from 1 or 2

4 gets internet from 1 or 2

how much should be length of sides? seems needs to do some algebric formulas.

or maybe I should keep 2 routers at each corner

one recieving and one broadcasting 

  @new8n 

Well, this new design brought a really important question. Why do you want to cover that area? You jumped from a central design that seems to be there are other CPEs 13km far, to a square design.

The one that you are showing is an entirely different network. Please explain your use case and what you want to achieve in the network so I can help you with the design.

  @new8n The "interference" referred to in this thread for your proposed central base station is that each CPE will be randomly transmitting / receiving (Time Division Domain) without any coordination.  When one station transmits while another is receiving, the "desense" will happen to RX with weak signals coming from afar (kilometers), but the noise coming from the radio a meter away.  I don't believe the TP-LINK has a way to coordinate the devices to all transmit and receive at the same time.  Some other vendors do have this feature, by using a GPS timing connection.   

As you get more complex and attempt higher speeds, there are other ways to obtain for a base station... including MIMO antenna arrays, etc.  There is product designed for this.  If you want to do this on the cheap, you might find clever ways with performance trade-offs.  Depends what your use or business case is.  Good Luck.  

  @RF_Dude I just touch on the coordination part, TP-Link uses Maxtream as a proprietary TDMA coordination mechanism in Pharos products.

Regarding the solution the use-case is not clear enough.