Curios on what message/sec rate depends on? Sure on # of planes but it is not linear or seems any logical dependency as it fluctuate sometimes wildly for same # of planes.
Number of visible planes.
Visibility of those planes to secondary radar.
How often the secondary radar decides to interrogate (which can in turn depend on the aircraft transponder type and the aircraft location)
Density of TCAS/ACAS-equipped aircraft nearby
Proportion of messages lost due to overlap (which in turn depends on message rate…)
tl;dr: there are a lot of factors that are very site- and traffic- specific.