According to my clan's webadmin, I can set it to 600 specimens at once, but who has a server that will easily support that?
I was once playing with sandbox mode and had total 6x players on one server and put the value to 150. After those specimen started to pop out and we practically got steamrolled, ping was increased atleast 50% and FPS went under 20.
So unless people want to have laggy server and massive ****, having more than 6+ players (ok, 10 is still reasonable) and increasing active specimen in the map might result everyone complaining how ****ty and laggy the server is.