The number of banks is just the number of outputs from the charger. For example, my boat has a 2-bank onboard charger. Independent charging of the starting battery and the trolling motor battery regardless of state of charge of the banks relative to each other.
In my case, each bank is a single battery. But if I had a 24 volt trolling motor I would have a 2 battery bank on one output from the charger and a 1 battery bank on the other output.