That's the big trick of a good LoRaWAN network. The ADR (Adaptive Data Rate) feature of LoRaWAN will make sure that nodes switch down to a lower Spreading Factor (higher datarate) and lower power such that it will still reach nearby gateways but not disturb gateway traffic from gateways that are further away.
In a controller environment, the network operator will monitor the gateway traffic and flag an issue. But within TTN I think we still have to use our own common sense. Within the Nijmegen area we are now looking at the people who have, or have planned, to place a gateway and determine antenna locations in such a way that we have a good coverage of the area. 4-6 gateways (with antennas on high locations) will be needed to cover the whole Nijmegen, Wijchen, Malden, Molenhoek area. More people will add gateways, resulting in a dense network of small cells accepting traffic from more nodes.