Don't know if it's the most ever, but many years ago a JAL 747 crashed with , I think, 523 people onboard. It was the one where the vertical stab blew off when the aft pressure bulkhead failed. The airplane flew around for a while before loosing directional control and crashing in Japan.
I think it was more or less a "commuter" flight, intra-Japan so they routinely would've had pax numbers like that. Only one FA survived IIRC.