Try to set
in main function. Before spawning ents at runtime and before level load this var must be set.
Default set is 10times your nexus.
I had this prob with grass models placed via seed map. So i set this var and all was fine. I used 30.000ents for testing, now around 12.000 grass models.
edit:
Guess u guys talking only about "hand-placed" models in WED / map editors. I was talking about runtime creation. So my post wont help much
Greets