5 y-wings is a herd, not a swarm. Kind of like 4 albino space cows for the imps.
It's all subjective really. You can fly 8 ties in 2 lumps of 4 (basically 2 largish base ships with 4 attacks each) and it's not much of a 'swarm' until it breaks formation. Swarms for me are more about how you fly, not what. But I guess it's important for what you fly too, because MOV.
If you wanna call 3 or 4 TIEs/Zs a miniswarm then go ahead. I'm pretty sure most people will try to fly that in some kind of formation instead of coming at ya from all over.
The "Rebel Swarm" as of Wave 3 was BXXYY
