Something on the order of 240. An FB attack, plus starvation/dehydration from Dwarves isolated by unnoticed bridge destruction by tantruming militia over deaths caused by the FB, plus a goblin axelord who tap-danced over the minefield, plus tantrum spiral from sum of all of the previous. The only survivors were one adult female and three unrelated children. And a vampire walled into a cell, but he did not help with the cleanup. He was just insurance.
Eventually I cheated and got the kids to make coffins, but they would not build them or bury the dead so the single free adult had to do that. She was depressed about it but she managed it in between tantrums. There was certainly plenty of good quality food and drink for the four of them, as well as a large if miasmic dining hall with at least some intact furniture. Before more migrants could arrive, the liaison came and made the fortress a barony. Rotting corpses in the hallways, four living souls, and all. I imagined him awarding her the barony, then slowly taking in his actual surroundings as he tenders his hearty congratulations and negotiates their trade agreement, and privately thinking to himself as he says their fortunes rise and fall together "Oh please please no ..."