We had to cut certain maps from RQuake because they had too many entitities and wouldn't load. Maps like SM82, Moonlite Assault. So basically, monsters are a significant factor on whether or not they can be run server-side.
That being said, some of the maps that have been tested and work have nearly 250 monsters. RQuake also has a gib and dead body filter to improve performance.
I hope that was helpful.
edit = I might add, check with the mapping community at Func_Messageboard.
http://www.celephais.net/board
I know of certain monsterous maps with 400+ monsters could only be run by AguirRe's enhanced capacity Quake engine that breaks numerous limits in engine capacity.
http://user.tninet.se/~xir870k/
I know over at func_msgboard they have a great degree of expertise of knowing the precise limits, concerns and such.