My guess is a total of 597 Pokemon. That would be 103 new Pokemon. I'm hoping most new ones are completely new (like in Gen 3, where the only ones related to others were Azurill and Wynaut).
BTW PokemonFreak44 and 42 chocolate, Gens 1 and 2 ended with 151 and 251 pokemon respectively, not 150 and 250.