Harvest Tree: Coins or Coupons? - How Many Ninjas Should Be Kept Ready?

On 2016-08-12 02:31:32 ｜ Show this Author Only ｜ Descending Order

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So, usually I chose coupons when the time came to harvest my Wishing Tree.
However, in these couple of days, I've switched to coins. After all, strengthening equipment is getting really expensive.
Am I doing that right? Or is hoarding coupons the way to go?

Also, so far, I'm awakening to the max only 7 characters:
My main team of 4:
- Azure Fang
- Kabuto
- Kakashi
- Hinata
And some extras:
- Neji
- Naruto
- Sasuke

That makes 7. . . How many Ninjas should I keep "up to date"?
I'm keeping Neji, Naruto, and Sasuke topped up because I really want to use them in separate teams for Ranked in the very near future.
I'm assuming that the answer is 12. . I just have to make sure that the 12 are all used by me in Ranked via Top, Middle, and Bottom, correct?