That's true enough from an in-game perspective, but the purpose of a "census" giving at least a rough idea of how many vampires are in the world is as out-of-character information for players and storytellers to have common ground in understanding the setting. Which is completely essential to any RPG because otherwise mismatched expectations can and will ruin the game.

If we say that there are "a lot" of vampires in London, does that mean 50? Or 500?

Obviously regardless of how many vampires there are in the world, there will be far more than 500 so...