The current 11.5 doc points out that there is no maximum value for -Mn parameter and the limit is based on available system resources. OEM was modified in 11.3 to remove the 512 limit.
At least since V10.1C (but not in V9.1E) doc says the same: "The maximum value for the number of servers that can be started for a database is limited by available resources of the operating system."
As I alreay wrote it's not true: the size of the user control table (a.k.a. the connection table) is limited by 32K - 1 where the size is (-Mn + -n + 3). It's meaningless (though Progress allows it) to start a database with the -n less than -Mn, the normal way is: -n greater than -Mn * -Ma. Hence the -Mn is at least limited by 32764 / (-Ma + 1).
BTW, the number of records in _ActServer / _Servers VSTs was changed from -Mn to (-Mn + 1) in Progress V10.
I guess the only changes in this area was done in Progress after 10.0B and before V10.1A. I have V11.4 installed and the tests did not find any difference compared to V10.2B regarding the -Mn limit.