If we're being faithful to the games : 2 episodes for each elite four and 3 for the champion. You don't need 17 episodes.
I think an example I have here for how long a "100% faithful to the games minus the little things you need to change for Pokémon to work better outside of a video game" Pokémon anime to go. Or at least how much you would actually need to consider when writing such a series:
I say that even though both Pokémon Origins and Ash's anime covered getting both the Pokémon and the PokéDex before the respective halfway points of their first episodes, the correct move would and should be making that... The
whole first episode. Why? You need to get through establishing:
- What Pokémon are.
- Giving a basic idea of what kind of world the Pokémon world is.
- What kind of town Pallet Town is and what kind of people its people are.
- What kind of kids Red and Green are and their relationships to each other and the adults around them.
- The personalities of the three starter Pokémon and why the boys might make their choice in starter.
- The first rival battle.
- Going through Route 1 and back to get Oak's Parcel from the PokéMart into Oak's possession.
- And depending on how much you want to adhere to Gen 1 or borrow from later games, you need to decide whether or not you're going to include Blue (I know, Green in the English translations and usually just Leaf. Don't... Just call her Blue).
- If you do decide to include Blue, then you also need to set aside time to establish her character and how she ties into the backstory of the two boys.
It doesn't sound like a lot, and maybe there is a way to streamline all of that so it can be covered in the same length of time as the other two anime, but I don't see how you can handle all of that in half an episode or less without it all just coming off as clumsy and rushed and "I don't give a crap". Not to mention you need to consider similar details like that for every episode, even if all that you're doing is a (mostly) filler episode on Route 12 as Red battles his way through generic Fishermen with underleveled Magikarp and Goldeen so he can wake up and battle Snorlax.
You
don't need to pull a DBZ and stretch out Red's Pikachu and Venusaur beating Giovanni's Persian and Rhydon into 10 episodes. I'm not saying that. And I admit that I'm maybe trying to put more story where there is none, and that that's bloating things. But from my perspective, even the least story driven Pokémon games need more than 12-24 episodes to cover all of their material. Especially since, as I've stated, the job of an adaptation is to flesh out on and expand both the world and the characters in that world.