From what I can tell, there are a lot more than one or two thousand of them.
The number of monsters in the swarm is taken into account in the monster ranking system in this world.
Even if they're D or C class, if 10000 small fries come together it's easily S class.
In a certain game in my previous life, ants three times as large as humans came out in large quantities.
But it's not necessary to be that big.
The monsters in this world are awfully good at moving around.
"Oh, that one is the queen.