Another big key factor is your plans for occlusion. if used properly, you can greatly trim down the primiatives and draw calls you have to deal with at any time. Making smart use of terrain can really play a big roll in this, as terrain via heightmaps has built in occlusion. So some well placed hills the player can easily run over but can't "see" over can create wonderful pockets of low draw calls and such without hindering the appearance of the world's overall size.
I think it's pretty standard to keep things under 1 million "primatives" at an given time. There are built in analysis tools that easily show you how much your scene is dealing with.
So take your budget into consideration. Figure out your target people per area, then start doing the math. Can quickly see what gaps you have for lea way. For example you want 100 players in an area fighting, and each one is lets say 8k with their gear (this would be pistol) as part of their total primitive count. That means 800k primatives are sucked up leaving you around like 200k for scenes, terrain, trees, etc. Then can just run around your scenes and see where you might have problems, at. Refine as needed.
Now with that in mind does it tell you how many polys you can muster for each weapon, not really. But realize the balance is to go as low as possible for any character visible asset, cause they are the only things that multiply. Meaning there could be 20 players on screen if so 10 extra polys each pistol means 200 more polys. if there are 100 people then that is 1000 polys to deal with. But if your game looks crappy people won't want to play it. So the juggling act can be helped with LODs. If the guy is 60 feet away it just needs to resemble a brown and black pistol shape. If the guy holding the pistol looks at his gun it best look good. So use of LOD really help here. Cause you cut down once again on the multiplying factor. If you once again use LOD and save 15 polys per gun over 12 feet, then you could be saving 1500 polys in a 100 man fight. While still having the player say wow nice details on weapons.
Also note when testing that when thinking on players that they drop from loading quickly as soon as they leave the "viewpoint" for x rotation they are dropped from having to render. This is a plus and negative. Plus side could have 300 people fighting behind you and your graphics card really doesn't care. Negative when you flip around and have to make all of those draw calls and such, your graphics card gets heartburn. With that said though use it to your advantage when planning out areas where lots of fighting will take place.