Zero.
The Unovians were the only people to have a Pokémon listed at number zero in their Pokédex.
Victini, the Victory Pokémon, which usually lived in the Liberty Garden near Castelia City.
Elio had yet to see one. They were psychic types, which meant that they could trick people into not seeing them. A part of the reason why some people believed there was only one in the world was because the rest hid themselves from visitors.
He made notes of the structure of the building once more and began to leave.
Maybe one day, he'd get to see them.