It's important for a realm to have a large neough population, such that you can actually find groups for quests and dungeons. I started playing on Nost and the population is immense, to the point where you are unable to do almost anything without grouping up.
I've also played on ED, which has very low pop, around 400, most of which are 60s. Again, it is impossible to get anything done, because you can't group up.
As far as the population on Kronos goes, as said before, you can use /who to check. However, something seems to be off, you could take a look at Kronos
Account Manager. There is a large discrapency between them. Sometimes the account manager says 2000 players online with 50-50 A/H ratio, while the /who says only 400 allies are online.
It also seems that /who caps out at around 460 which is a bit weird. In any case, I have had no problem at all finding groups for difficult quests, while also being able to solo the rest of the content. Regardless fo the actual numbers, the population of Kronos seems to be good enough.
I think retail vanilla had abount 2k players total per realm, but I'm not sure.