The gym mechanism has changed drastically since Pokemon Go first launched, so here is an updated answer for the new mechanics:
Is it possible to put more than one Pokemon in a gym?
In a single gym, no, you can only put one Pokemon per Gym. But, you can have up to 20 Pokemon defending different gyms.
Some other things to keep in mind:
- Gyms only have 6 slots for Pokemon. If all the slots are taken, the button to add a Pokemon will not appear.
- You can only put Pokemon in either neutral (grey) Gyms, or gyms of your team colour i.e. Red (Team Valor), Blue (Team Mystic) or Yellow (Team Instinct).
- You will need to battle and knock out Pokemon defending a gym of a different colour in order to return the gym to 'Neutral' status and add a Pokemon of your own.
- Pokemon are knocked out when their Motivation (CP Heart) reaches 0. You may need to battle multiple times to knock Pokemon out of the gym.
- Gyms will only allow a single Pokemon with the same Pokedex number.
- For example: You can't add a Charmander when there is already a Charmander defending the gym.
- This includes alternate forms and costumes - you can't add both a Raichu and an Alolan Raichu to the Gym
- Note that Pokemon evolutions don't share a Pokedex number, you can add your Charmander when there is a Charmeleon or Charizard on the gym.
- The Gym's 'battle order' is based on the order in which you added a Pokemon. First Pokemon added, first to battle, and potentially the first to be knocked out.
- You can earn coins for leaving Pokemon in Gyms. Pokémon accumulate one PokéCoin per 10 minutes when they're in a Gym.
- You can earn a maximum of 50 coins per day.
- Coins are awarded when your Pokemon is knocked out of a gym
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