How many heads do hydras have and what is the significance of this unique feature in their mythology and biology?

Hydras are mythical creatures with multiple heads, typically depicted with nine heads. In mythology, the significance of their multiple heads is that when one head is cut off, two more grow in its place, making them nearly impossible to defeat. In Biology, this feature is not seen in real animals, but it symbolizes the regenerative abilities and resilience of the hydra in popular culture.