Keeping Primitive Counts Under 100,000. ok now i can see poly count of environment. And what about dynamic obj such as players. Let's take 1000-1200 poly count of character. We are using basic development sycle - 3d sculpting - painting - retopo - finishing with uv maps (diffuse, normal, spec and alpha maps) - job done. All characters are consist of 50 bones.
720 000 characters poly count + 100 000 or 200 000 for environment = 820 000 or 920 000. less than million. We are not talking about fx effects. Most of engines can handle 6m of poly. Lets return to the start. How many polygons can HE handle without loosing fps? 
It is really irrelevant to how many polys the engine can handle. If you are using DirectX as a renderer you follow the rule of thumb that all game developers follow which is "As low as you can go". It is up to you how detailed your characters are as well as your world. It is always good to have a pretty game, but understand you can achieve that with a low poly count by taking advantage of baking normal maps by modeling with programs like Zbrush and Mudbox. The characters in Gears of Wars are near 10k -15k polys, if you are exceeding that for an mmo per character your may need to find a way to optimize your characters etc. Here is an idea of what some games are using polycount wise (it varies greatly per genre of game):
Gears of War, Xbox 360, 2006
Wretch - 10,000 polygons with diffuse, specular and normal maps
Boomer - 11,000 polygons with diffuse, specular and normal maps
Marcus - 15,000 polygons with diffuse, specular and normal maps
GTA San Andreas, PS2, 2004
Characters - 2,000 polygons with 1 256×256 8bit texture
NPCs - 1,200 polygons with 1 256×128 8bit texture
Halflife 2, PC, 2004
Alyx Vance - 8323 polygons
Barney - 5922 polygons
Combine Soldier - 4682 polygons
Classic Headcrab - 1690 polygons
SMG - 2854 polygons (with arms)
Pistol - 2268 polygons (with arms)
Halo, Xbox, 2001
Masterchief - 2,000 polygons
Metal Gear Solid 3: Snake Eater, Gamecube, 2005 (Small mistake here, it's not on GC but PS2)
Snake - 4,000 polygons
Resident Evil 4, Gamecube, 2005
Leon - 10,000 polygons
Jak & Daxter, PS2, 2001
Jak - 4000 polygons
Jak II, PS2, 2003
Jak - 10,000 polygons*
Lost planet, X360/PC, 2007
Wayne - 12392 polygons (but finally 17765 polygons for compatibility with motion blur effect)
VS robot - 30-40K polygons
Background - ~500K polygons
Peak number of polygons per frame - ~ 3 million**
Dead Rising, X360, 2006
Peak number of polygons per frame - ~ 4 million**
The Legend of Zelda: The Wind Waker, GC, 2002
Link - 2800 polygons
The Legend of Zelda: Twilight Princess, GC/Wii, 2006
Link - 6900 polygons
Super Mario Sunshine, GC, 2002
Mario - 1500 polygons
Levels - ~ 60k polygons
Dead or Alive series, Xbox, 2001-2004
Character - ~10k
Vitua Fighter 5, Arcade/PS3/X360, 2006
Character - ~40k with diffuse, specular and normal maps
Background - 100K - 300K polygons
Medal of Honour: Allied Assault, PC, 2002
Character - 4096 polygons
Project Gotham Racing 3, X360, 2005
Cars - 80K-100K polygons (interior + exterior), damages add between 10K and 20K more polygons per car
Gran Turismo 5: Prologue, PS3, 2007
Cars - 200K polygons (probably interior + exterior)
Midnight Club, Xbox360/PS3, 2007
Cars - 100K polygons
Gran Turismo 3, PS2, 2001
Cars - ~2K-4K polygons
Gran Turismo 4, PS2, 2004
Cars - ~2k-5K polygons
Also if you must know the number, make a sphere that is 62k triangles, and make multiple instances of it till you fell it is sluggish for you min requirements.