While one could avoid TTN’s Fair Access Policy when simply not using TTN’s community network, one should surely come up with one’s own limitations then.
If one hammers a, say, 1% maximum duty cycle all day long, then if one’s devices somehow never transmit simultaneously and if no one else would be using the same radio spectrum, then a gateway could handle 100 devices per frequency/SF. But:
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Of course, nodes will transmit simultaneously (including partially overlapping transmissions, where one device is still transmitting when another one starts).
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Of course, devices from other networks will share the same radio spectrum (including networks using other LoRa technologies).
I feel the Fair Access Policy has a simple but sound mathematical basis:
The above does not take the density of the network (the number of gateways in an area) into account. But every network, including private networks, needs something like the above.
Any use case that needs more should look into using different technologies, or set up a very dense network where nodes use a very low transmission power, hence the number of nodes per gateway is decreased. Such dense network could (should!) still be connected to TTN, as the radio spectrum is shared anyway, and connecting it to TTN allows ADR-enabled TTN devices to lower their transmission power as well, decreasing the chances for collisions with the devices from the owner of the gateways.
See also What happens if someone does not follow the guidelines? - #15 by arjanvanb.