OK, this may be a simple old man's brain fart. And by "may" I mean pretty certain.
I have Vivien, Monster's Advocate and Lathliss, Dragon queen in play. I activate Viv's last loyalty ability. Tap some mana and play Broodmate Dragon. Now, before it resolves and ETB, triggering Lathliss and itself, Viv's abilty is trggered as a spell being cast. I search up Scourge of Valkas and put it onto the battlefield. Doing that triggers Lathliss and itself. So I now have a spell (Broodmate) and 2 triggered abilities waiting to resolve.
The Broodmate spell is still on the stack when the Vivien trigger resolves, so the Scourge & Lathliss triggers go on the stack on top of that and resolve first. You control both sources, so you choose which trigger to place on the stack first, and then they resolve in reverse order. So if you stack the Scourge trigger first, and then the Lathliss token ability on top of that, the token will enter and trigger the Scourge again, and both would count all 3 dragons and do 3 damage each when they resolve.
After that the Broodmate resolves has 3 total ETB triggers (itself, Scourge, and Lathliss), and each token will again trigger the Scourge. If you stack Scourge trigger from the Broodmate first, then it will resolve last and count all 6 dragons (Lathliss, 2x Lathliss Token, Scourge, Broodmate, Broodmate Token). And whichever token you stack last will enter first and have its Scourge trigger resolve before the 2nd token is created, so it would only see 5 dragons.
If you do it right you can end up with 5x Scourge triggers, resolving for 3,3,5,6,6 damage.
I have Vivien, Monster's Advocate and Lathliss, Dragon queen in play. I activate Viv's last loyalty ability. Tap some mana and play Broodmate Dragon. Now, before it resolves and ETB, triggering Lathliss and itself, Viv's abilty is trggered as a spell being cast. I search up Scourge of Valkas and put it onto the battlefield. Doing that triggers Lathliss and itself. So I now have a spell (Broodmate) and 2 triggered abilities waiting to resolve.
Spell first? Triggers first? My choice of order?
After that the Broodmate resolves has 3 total ETB triggers (itself, Scourge, and Lathliss), and each token will again trigger the Scourge. If you stack Scourge trigger from the Broodmate first, then it will resolve last and count all 6 dragons (Lathliss, 2x Lathliss Token, Scourge, Broodmate, Broodmate Token). And whichever token you stack last will enter first and have its Scourge trigger resolve before the 2nd token is created, so it would only see 5 dragons.
If you do it right you can end up with 5x Scourge triggers, resolving for 3,3,5,6,6 damage.