We've had up to (6) 2MP cameras streaming through 1 bridge pair at 6FPS. I'm not comfortable with much more than that simply as a gut feeling.
We had a site where 30 IP cameras were on satellite buildings circled around 1 center building. Multiple Ubiquiti bridge connections with about 1 to 3 cameras on each bridge streaming to the center building. Bridges on the center building connected together on a switch and then all streams went across another wireless bridge to another building with the VMS. They would have intermittent problems with streams dropping in and out. Sometimes rebooting the switch fixing it, sometimes the VMS, sometimes the bridge.
We replaced the wireless bridge going from the center building to the VMS building with a fiber connection. That cleared up the issues.
According to bandwidth calculations and what we saw on the VMS, bandwith on the bridge connection between the central building and VMS building was plenty enough. What I think was the bridge pair going from the center building to the VMS building just couldn't handle 30 different streams.