- Hope that most of your clients support 5ghz.
- Hope that most of your clients support DFS channels
- Deploy DFS channels and make the 5ghz 6db stronger than the 2.4ghz band.
EDIT:
Honestly, "seeing" 100 other networks is not really a dealbreaker. RF can travel hundreds of feet, especially in the 2.4ghz, so they can be observed with laptops, but less with mobile devices. If clients are actively connecting to those foreign networks AND sending data AND YOUR clients can see the traffic, that is what could be problematic. Most likely you can see these networks because the access points are powerful, but the clients might not be powerful enough to create more cochannel interference for you.
If these are 5ghz networks, the cochannel interference from these networks is less of a problem, because it does not propagate as far as the 2.4ghz band. This gives you an opportunity to deploy on 5ghz to support your clients.
If these are instant APs, you would want to keep an eye on the RF utilization on any channels that you choose to deploy on by looking at the front page. On controller-based access points, the dashboard will also give you an idea of what the RF utilization is on any access points you choose to deploy.