2 wings (1 set)- low class devil
4 wings (2 sets)- middle class devil
6 wings (3 sets)- high class devil
8 wings (4 sets)- Ultimate class devil
10 wings (5 sets)- Satan class devil
12 wings (6 sets)- Satan class or on par with the devil kings.
Sosuke will be stronger than usual in every rank thanks to the goddess purifying his blood but won't be an overpowered boost, just so he will be able to hang with the plot armor of the main cast.
There is also Akeno, her fallen mode can just be explained as a perfect fusion between her fallen and devil traits that temporarily overboost her magical capabilities as a fallen and devil. Her wing equivalent to power doesn't exist, her wings are practically for show of fanservice.