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.
or every entity, around 1.5..2.5 KB memory is reserved from the nexus, dependent on its bones size.

Example:
Code:
max_entities = 1000; // alows up to 1000 Entities. 
level_load("test.wmb");
wait(2);


See also:
max_particles, num_particles, num_vismappolys, num_visents, num_visentpolys, level_load, nexus


alternatively, you could just have used the SEARCH function:
http://www.coniserver.net/ubb7/ubbthreads.php?ubb=search

heck, even google tells you what to do:
http://www.google.com/search?hl=en&q="not+enough+entities+reserved"&btnG=Google+Search&aq=f&oq=

"Sometimes JCL reminds me of Notch, but more competent" ~ Kiyaku