Legendary means remarkable enough to be famous or very well known or described in, or based on legends. So using the definition itself, we can assume that it's a Pokemon that is famous enough to be written in lore and legend. It's probably so rare that most people aren't even sure it exists, and if it exists, it probably exists as a singular entity.

At least the books I've read that include Legendary Pokemon speak as though they don't have multiple instances of them like most of the Pokemon we own.

Are you trying to catch one?