From the manual

max_entities
The maximum amount of entities in a level; must be set before loading of the level. The default value is nexus times 10.
Range:
0 .. unlimited (default: Nexus * 10)
Type:
var
Remarks:
Is to be set before level_load.
For every entity, around 1.5..2.5 KB memory is reserved from the nexus, dependent on its bones size.
Example:
max_entities = 1000; // allows up to 1000 Entities.
level_load("test.wmb");
wait(2);