After the existence of the enemy Heroic Spirit Army was confirmed, Tohsaka Rin couldn't snap out of it for a full three minutes.
This was too ridiculous, the kind of absurdity that made headlines! A Heroic Spirit Army? Hundreds, thousands of Heroic Spirits? How could that be possible?
Having seven Heroic Spirits appear in one Holy Grail War was already a miracle born after exploiting a loophole in the fabric of the world, representing the summit of mysteries accessible and controllable by humans. And just these seven Heroic Spirits alone were sufficient to trigger a conflict that could be called a "war," which highlights their might and indicates the extent of their impact on the real world.
So what about five thousand Heroic Spirits?