Jonathan Hui can you make a stab at answering this?
This link talks about device limits.
https://openthread.io/guides/thread-primer/node-roles-and-types#device_limits Basically 32 (FTD) router limit and 511 End Devices per router (That is a very large mesh, 16,352 devices ), as the number of devices moves up, how many linux or RPI OpenThread Border Routers (OTBR) would be optimum?
For Wifi redundancy I would want 3 OTBRs for any large mesh network as a minimum, wondering what would be a sensible maximum. I am not sure if connecting 2 to 3 (FTD) Full Thread Devices as routers physically to the OTBR makes any sense. As far as I can tell no (FTD) router actually uses Wifi to connect to a border router, so all Wifi / Thread interaction works physically at the linux or RPI OTBR.
(I guess the Particle Argon is an exception as it has separate Wifi and Thread antennas so theoretical an Argon could connect using Wifi to an OTBR)
Any comments Jonathan? Do most thread networks simply have one OTBR?