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. 

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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

**Sources:**

- Personal Experience
- [imore.com - Pokémon Go Gyms: How to defend, attack, earn coins, get stardust][1]
- [Pokemon Go Wiki - Gyms][2]


  [1]: https://www.imore.com/pokemon-go-gyms
  [2]: https://pokemongo.fandom.com/wiki/Gym