Given the Alliance and the Horde had a full-scale war spanning two islands (Kul Tiras + Zandalar), Eastern Kingdoms and Kalimdor (and Broken Isles if you include those Allied Races), we're talking minimum several million people combined. Oh and of course I'm excluding those who died from the burning of Teldrassil obviously.
For context, I'm playing through BFA for the first time this summer and I'm doing the Alliance side first. Given the scale of just Horde casualties plus the later divide within the Horde, they had to have lost over 70% of their fighting power during the Fourth War. On the flipside, Stormwind depleted its military so quickly, Anduin had to conscript farmers.
Lorewise, the Alliance won both battlefronts and headed off The Barrens being the third planned battlefront. We know from her commentary after the fact Sylvanas wanted the war to continue for a bit longer. Siege of Orgrimmar 2.0 probably would have won the war for Sylvanas had it happened. By the time Saurfang challenged Sylvanas outside Orgrimmar, Anduin was only able to spare a single regiment and they were there mostly as a show of force. If Sylvanas didn't accept Saurfang's challenge and just blighted everyone outside Orgimmar--obviously killing Anduin in the process--it would stopped being a war of attrition and become a war of extermination. In hindsight, it probably would have made The Jailer's plans for Anduin much easier had he died then and there.
Speaking of, we probably won't know how many people died while the Scourge were rampaging across Azeroth and more so given the events of Shadowlands spanned two years. The Alliance and Horde forces were already devastated from the Fourth War so...yeah. Aside from the Jailer, the only force on Azeroth that became stronger because of the Fourth War was the Ebon Blade. They recovered the bodies of soldiers at every battlefront and even raided cemeteries for bodies that were later raised as Death Knights.
I can imagine Alliance Death Knights telling their living allies they'll make sure Sylvanas can't raise them knowing they had orders to bring back bodies to be raised as Dealth Knights:
Newly raised Void Elf Death Knight: I remember being killed in Nazmir. How am I here?
Night Elf Death Knight: You were chosen for a greater purpose. Welcome to the Knights of the Ebon Blade, brother ^_^
...Sweet irony. LOL.