The limits I have seen had more to do with the alerts/second generated from the sensors than the number of sensors or models themselves. I have seen deployments of 50+ per manager with well tuned policies and moderate throughput and deployments that couldn't handle more than 10 per manager.
McAfee used to prescribe no more than 50 per NSM, but that was pre-8.1.
Yes the limit does appear to depend on the volume of alerts generated by the sensors. The 8.2 best practices guide lists this limit as "1500 alerts per minute from all the Sensors configured to it" which works out as 25 alerts / second, which is very low for the high end model sensors.