You need to make a selection of 12 ninjas even if you only use 4.

Since you only chose 10, the game system thinks that the 2 that you did not choose are actually defeated. That's why you could claim the "maximum of 6 and 3" one but not the "maximum of 1". Just choose another 2 more tomorrow and you're gold. Since these 3 can only be claimed once throughout the event, no harm done.