Come meet my Sentinels, or go have a cup of tea with the Harvesters. Either way, we RP, all the way. Sentinels be good (when they work) Harvesters be... well Nom noming people.
Server stops spawning them at... Well it used to be 150 players... Not sure what it is now, but it's proportionate.
E.g. at 100 players there will be loads of NPCs, at 125 not many, at 149 barely any.
This is to keep server load down (duh).
Only way around it is missions.