1 million is used a lot to say how many worlds does the emperor rule. it is an estimation because nobody knows for sure. The war in W40k is constant and winning a crusade in one sector of the galaxy can bring many more worlds into "the fold" while at the same time loosing entire sectors to aliens, Exterminatus, Heretics...
Of the elder races ( Eldar, Necrontyr ) it is often said that they ruled the entire galaxy at the apex of their power. Again, no precise numbers... So I am afraid that you wont get your answer any time soon.
As for present day Eldar there used to be a map of all of their exodite worlds and their maiden worlds and the approximate placement of the remaining Craftworlds somewhere out there on the big scary web.
It is something GW made so if you search for long enough you will probly find it.
EDIT 26.05-2014 ( wouldn't fit into comment )
@ete those are to date the most detailed maps of the races in the W40k universe. im guessing that there may be some newer ones in the different codex books. not sure tho. as I don't have anything after the 5th codex upgrades. And even those maps cant help you much... today they are like that, but you never know if there is going to be a 14th black crusade that is going to turn everything upside down again. Also after the 13th crusade many of the empire worlds around the eye got snatched up by chaos and various other predators. Loads of Space Marine homeworlds got destroyed either in that or when the Tyranid started advancing. so rough estimates is all you will ever get as the planets change hands on a daily basis. The galaxy is vast, and the war is eternal!