Try going to www.serebii.net and find it there, because I dont know if you are asking about the national dex or the regional dex.
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Number 151 is manaphy
I think he is number 151
number 151 is mew. If in the regional pokedex its manaphy, which can be found in a book on route 212 in the mansion.
#151
#151 of the Sinnoh 'dex is Manaphy, you can see it in the Pokémon Mansion, reading the book.