Populations with various margins of error:
Kingdom of Aylsfyn - Yellow - big island - 1,090,000 Humans
Veden - Blue - arable river source with some hills - 63,400 Humans
Akataneka - Pink - swamps, coastline and other arable bits - 940,000 Humans
Romeq-Intik - Green - a number of small islands - 8,500 Vilitii Elves
Thernopolisian Empire - Purple - lake and streams, protected by hills - 2,100,000 Humans with other humanoids
Dominion of Ordov - Maroon - fertile river delta and coastline - 4,020,320 Humans
Graeg - Light Blue - large expanse of cold hills, coast and fjords I think - 100,000 Humans
Saviana - Orange - one small island - 150,000 Savian Elves
Kingdom of Fenia - Dark Green - coastline between two rivers, quite arable - 250,000 Humans (with fantastic facial hair)
Lumenor - Light Grayish Brown? - Large territory bordering three big conjoined lakes on the edge of the steppe - 2,000,000 Lumen Humans
United Tribes of Heima - Dark Blue which is not darker than Veden - Large territory up in the tundra with coastline - 100,000 Humans
Reinnam - Dark grey - large hilled and mountainous peninsula, volcanic (which I would assume makes it nice and fertile) - 200,000 Saphri (mostly ginger Bruce Lee clones)
Azu Warbands - Red - Some hills, river and lakeside - 60,000 Azu (decentralised angry cat people)
Uraka Federation - Light Green - Hills, river and lakeside - 480,000 Kharis (bears that are not cavalry), 696,000 Humans, 24,000 members of other races.(1,200,000 total)
Zivaria - Blue which is I suppose between the shades of Veden and Heima - Small island, but water-dominant species - 51,000 Zivar (Shark elves)
Adrathielish Empire - Dark Pink - Large chunk of fertile delta region with river and coastline - 900,000 Humans with traces of other races

[url=http://www.youtube.com/watch?v=09KL2HUXE6Q&t=37]

As for you, Attilla, I'd say you could get away with at least 1 million with all that space to share, but there may be other viewpoints.

EDIT: Also note that current rate of population growth might be a better indication of how well the various populations are being sustained, but I'm no demographer or anything, there's a whole bunch of things. This is just a snapshot to give you an idea of what other people chose to start with.