When it comes to raiding, both Horde & Alliance are able to be in the same raid together now. The Alliance population is pretty much fine! Sure it’s not as big as Horde, but it’s not at a point, where there are no Alliance around.
I would say, from a guess, the game is maybe 60% Horde and 40% Alliance.
Silvermoon is the largest Alliance server, so if you want to make a character, you can go there. But also the game is cross realm as well, so you will see other players from different servers, in the phase that you will be in for example. So the game is never really empty.
Not a thing anymore like others have said but it was never really true before the changes anyway. “impossible” was non-sense. Took a little longer to form groups during “off-peak” hours, sure, but not impossible or even difficult.