There's no solid answer (which is a problem). However, we can make an educated guess of there being at least a few survivors. When the Reapers take the Citadel, there only immediate concern is moving it to a safer location, rather than taking the time to systematically harvest everybody on it. My guess is, they entered the system in force, smashed the defending Citadel fleet, took out strategic positions for mounting a defense on the station itself (C-Sec HQ, any ground-based artillery etc.) and then proceeded straight away to move it without bothering about anything else. Casualties in areas specifically targeted would be extreme, but otherwise there may not have been that many at first.
Obviously, casualties on the Citadel will be far greater in the Destroy and Synthesis endings, due to the station breaking apart. However, we don't know just how advanced the life-support on the station is. I'm guessing for something that size there's no one system that, if it loses connection, kills everything on the station. More likely, life-support is more localised, so it's plausible that even on the fragmented pieces of the station, many people could still survive at least long enough to be evacuated.
So, I don't think everyone died. In fact, it's possible that, especially for the Control ending, casualties on the station itself could have been relatively low.