A total of 421 entities. Thats in the limit.
Here is the info from Tommy 14's error guide:
HOST ERROR: PF_precache_model_l:'sprites/spritename' overflow
HOST ERROR: PF_precache_model_1:'models/modelname.mdl' overflow
Precache is a game memory problem. It often could be you have a bad path/name for a sprite/model/wav. But it can also be that you have too many kinds or too large sprites, models(400 obj limit?), wav sound files(limit<3min) or entitys(data limit ~0.5M) and have overfilled the space setup for them. (see also SVC bad below.) Sometimes combining entities into fewer helps, but usually you will have to simplify to fewer kinds of sprites/models, or smaller sprites/models/wavs. Occasionally this error happens when you are using a corrupted or non-standard model/sprite. Using other folk's prefabs are often hidden reasons for this problem.
Sometimes the "too many wads" problem sets this off, or having a brush textured with AAAtrigger texture visible in the map will cause this problem.