When starting a War, there would be a maximum amount of Players that can fight depending on the size of the Team:
It goes in multiples of 5:
10 vs 10 / 15 vs 15 / 20 vs 20 / 25 vs 25 / 30 vs 30

If there are too many people in the Team, the one with the weakest Monsters, will become an observer, allowing them to see how the War is going on, but not able to take part.
The way to make sure you will always be chosen to fight, is to level up your Monsters and place some great Runes on them, to make them as powerful as possible.
This way, you will always be involved in the fight!