Error: too many entities
That means you have too many entities visible/see-able at that time. There are two likely causes for this: either you aren't running VIS, or your map has huge amounts of entities visible at a time.
Solution: run VIS with the -full parameter, use line-of-sight blocking (aka. vis blocking), combine entities of the same type (without special, brush-wide properties) in the same area - but only in the same area.