How many Pokémon is it possible to encounter in Pokémon Black?

Question

Your PB Pokédex says:

This is a list of the Pokemon from the Unova region in Pokemon Black & White. In a similar fashion to the Hoenn games, in Black & White the native Pokedex only contains the 156 new Pokemon from Unova. You can later obtain the National Dex which includes the other 493 Pokemon.

Does that mean after I get the NP I can find a "wild Pikachu?"

Answer 1

In Black, you can only obtain 156 Pokemon in the Unova Dex. Pikachu doesn't appear in Unova, so you cannot catch it. A National Pokedex just means that you can now record data for all Pokemon.