"How did professor oak know there were 150?"

Most, Pokémon, you could reasonably understand professor oak knowing about but like he said there were 150 when he sent ash out on his journey but at that time no one knew about Mewtwo, so how did he know there were 150 and on the subject matter, you really would think professor oak would be way more interested in togepi considering at that point in the canon everyone only thought there were 150 Pokémon. I know this question can’t really be answered, but I love thinking about the inconsistencies of the anime via /r/pokemon https://ift.tt/9xjlU8z