There can be millions of Beastmen, like mentioned above regarding Skaven they have their own food source - other Beastmen.
My guesstimates for the Dwarfs - i'd say at least a couple of million spread throughout the world. Just 1 millions seems too few for them to be such major players in the Warhammer World.
Skaven - their population varies massively. When Skaven hordes erupt onto the surface its due to population pressure. The Skaven population would then range from (guesstimating here!) a low of 15 million to a high of 30 or so million. They have the entire world to live in (albeit underground) so only a huge population would force them onto the surface. Also, most Skaven are basically, just rubbish so will die in droves.
I'd assume Ogres are in the region of 2 million. A significant enough number for them to hold their own in their region of the world and still have Ogres go off to see the sights and fight the fights elsewhere in the world.
I can't remember if I'm forgetting any races here :P