What’s a recommended triangle count for a game similar to Left 4 Dead where there will be 50 - 100 enemies on screen at once? Looking at the SK Manequinn in 4.9, it has like 40k triangles which seems kinda overkill to me…
Honestly, it can vary a lot.
I guess the question should be, “what’s a reasonable poly count for a 2015 / 2016 game”? Like back in the 90’s with Quake 2, it was around 800 tris, then during the CS era, it was 1500… Half-Life 2 era was around 4000’ish (I recall Alyx having that many)… ?
I’m guessing 8k to 12k for a character in 2015 is considered “low poly”?
How can you check how many triangles are being used per mesh?
Impossible to say, the only way is to iterate and continuously test. It’s easier to go higher now and go lower later, than do it the other way around.
I was testing for fun zombie AI with 4.9 Manequinn and with GTX450 it run smoothly even with ~100 enemies on screen (with GTX970 even better!) 
But I guess 20k will be good enough to keep quality and performance.
I always think like that: the lower the better. So create meshes that are looking good + have the lowest possible tri count -> also keep in mind that the drawcalls are also important (materials,…) 