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.