General

7 coins from red leaf high, 2 coins each for every absolute belt quest beside the first one that gives you the belt, and I think that's it, I don't think the event that gives you coins for spending nx is ongoing anymore.

Something interesting to note though with the absolute belt, it gives you a new quest every time you reach a new level range (if you accept the quest between lvl 31-50 and level to 51, the next quest will become available even if you haven't finished the one before it yet), so if you upgrade it as you level you can get 3-4 upgrades in one day. So new characters can get 10+ black friday coins in 1 day if you're diligent.

7 coins per successful run.
1 from 1st Kanna/Hayato
2 from 2nd Kanna/Hayato
4 from beating Mori Ranmaru

You can get 18 coins total from an Absolute Belt, 2 per upgrade excluding the first one, but the event ends today. If you can level from 13 to 130 today, that would get you a maximum of 10 coins or 12 if you have already started the belt.

rlh, abs belt, nx