If you weren’t aware, banks consist of metadata (mixer and event structure, etc.) and sample data (your source audio). When loading a bank, unless you manually choose to load the sample data, only the metadata is loaded. Instead, when you create an Event Instance, FMOD will load the sample data on demand. As a result, it’s unlikely that you need to be concerned about banks taking up too much memory - I would recommend assigning your events to banks appropriately so you know which banks to load ahead of time for a given section of your Unity project.
For more information, I would recommend giving the following thread a read: Is it possible to load a Bank from an EventInstance or EventReference?
As well as taking a look at the Sample Data Loading section of the Studio API guide.