I appologize for the slight necromancy here, but in case this question ever comes up again- Cause I know I'm not the only one poking around with mapping...I just wanted to answer this cause I don't leave the NS general discusion forum often and don't see posts in this forum more than a few times a year.
Item one- yes there is an entity limit. any given NS map can have no more than 400 process entities.
Item two- generally, this won't effect performance unless the entities produce many effects all within VIS rendering at the same time for one player. Ie: like 40 doors in a straight line all visible to the VIS engine simultaniously. The server won't have a problem running it, but the clients will therefore stressing the performance between eachother.
Item three- not all doors, plats, etc are just one entity. Any given simple door will likely end up being more than one entity. NS requires that virtually all doors have an external trigger. Therefore the door eats 1 entity, and the switch eats another. (if the door needs a switch on both sides, make the door a func_train and at the time of constructing the switches, group them all to the same entity with a "delay" and "wait" key pairs timed with the operation of the door/plat. This will save you a few entities off the count.)
Item four- If a door or platform/elevator is made of many brushes, (ie a platform consisting of a transperent floor grating, a 4 brush solid frame around it, railings, and some machinery brushes that move with the elevator-) The whole "group" only counts as ONE entity. You can even have different brushes in the same mobile entity have different rendermode, renderfx, and different renderamt key pairs.
Item five- standard light entities and texlighted surfaces do NOT count in the entity cap.
they only count in this total if they are triggerable, or use custom apperance flags and require a "targetname" key.
Those are all the specific answers I can think of at the moment in regards to this topic. If there are any other things you'd like to know, PM me.
One thing to note on triggerable lights... it significantly increases the map size and compile time. I'm not certain if it hinders performance much, but it would be unwise to have multiple triggerable lights that do not all trigger at the same time in one room and cover much of the room. I hate toggle lights they still have their place though.
If you want a switch, the door will be more than one entity. NS might not handle this well, but I believe in some mods anyway doors can be set to trigger when they are touched or shot. (even most doors that open when you approach use a trigger)
I have never hit the entity limit, and have only heard it mentioned once or twice in threads. Odds are if you get that many entities without getting a max packets error (I think thats what it's called, the one where your map is too big) you are trying to do such complicated things that you will probably be experiencing problems anyway.
There should be a nifty entity count button, probably under tools or map properties... it's been a little while.
The only other downside of having lots of entities is keeping track of them all. It's easy to have things triggering things they aren't supposed to because you messed up the names.
If you have the doors in seperate locations, and they are relatively simple (button and door) you shouldn't notice any problem having alot (like dividing all hallways and rooms in a given ns map with doors).