Have an integer for each player in an a array.
Then simply:
Have a trigger or two:
Event: A unit finishes training
Event: A building finishes construction
Actions:
Set tempInt = Player Number of owner of trained unit/building;
Set PlayerUnits[tempInt] = Player[tempInt] + 1;
Then another trigger for checking when a unit/building dies:
Event: A unit dies
Condition: Boolean(Unit classification not equal to summoned unit)
Condition: UnitType of unit not Equal to Blood Forge
Actions:
Set tempInt = Player Number of owner of triggering unit;
Set PlayerUnits[tempInt] = Player[tempInt] - 1;
if (playerUnits < 1)
defeat player[tempInt];
Or, set the food given by the blood forge to 0, then check the food property given and used by each player, if both are 0, defeat the player.