I believe the maximum is 25 per map. I'm well acquainted with it since on the mission I'm working on, I've hit it on all but one interior map.
There is a work around though. If you don't need the NPCs to actually be able to be spoken to via objective or "default text", you can substitute NPC Groups. You can either reskin neutral targs or reskin federation NPCs.
The benefit of the targs is that they're neutral and won't fight, and also that you can place only a single one. The benefit of the federation reskin is that they'll fight with you, and also they will use the Fed beam in animation when they appear (the targs just fade in which also may be preferable sometimes).
There is a per-map contact limit and its about 20-25, I'm not sure precisely.
Like it was said, re-skinning encounter groups can be a workaround, but also keep in mind there are costume limits for the entire project. I think that is around 40-50. But that really only matters if you want to make a lot of unique looking contacts/groups.
If you just want to have friendly Orions, for example, you can use the standard skins Cryptic built for Orions and reskin Feds or neutral groups as long as you want (until you reach the encounter limit of course).