How can I get more Ninjas?

You can have up to 5 Ninjas.

In order to unlock additional Ninjas, you need to make progress on the leaderboard, where you can also see how many Ninjas you will unlock.

Alternatively, you also have the possibility to purchase additional Ninjas from the battle loadout screen, but you need to unlock at least one of them before the option to do so becomes available. Purchased Ninjas will remain available until the next event starts.