This game is going to have tons of content. Nintendo has confirmed there are 200+ Pokémon. (We have seen less than 100 so far, and most of those we have barely seen in action.) Some have variants too, like the many wing patterns of Vivillon or the gender colors of Hippowdon.
Each of the 200+ species has a variety of behaviors, including four special behaviors, ranked in rarity (one star, two stars, three stars, four stars).
So that's more than 800 unique shots you will need to take to complete your Photodex.
You will have to figure out how to make many Pokémon appear, and then figure out how to elicit the desired behavior. Sometimes you will have to lure them to a certain area, or coax them into interactions with other Pokémon. You might have to use certain items, cause environmental reactions or return at a different time of day.
Nintendo has also confirmed that Pokémon will start to exhibit new behaviors as the game progresses, so even if you thoroughly explore one level before moving on, you will still need to return later.
This game is going to be amazing.