Is there any way (macro, script, addon, command, API method?) to check how many online players are in the realm you're connected to?
The Census addon provides lovely minimalistic graphs/statistics about the realm population (classes, races, lvls, guilds). It does that by performing a series of /who so that it covers the whole population.
Each scan takes about 2 minutes (on my server) and at the end you get a small chat report that is like
From which you can conclude that there are 700 characters online.
Have you tried /who? I think I remember that working
Edit: I found a simply LOVELY site after some googling: http://wow.realmpop.com/ It looks exactly like what you need. If you need something more than a website, I think this is where my help ends