question

You can access the bays directly from the storage object.

Storage.Object Rack = <Insert reference to object here>;
int BayNum = Rack.bays.length;

As each bay might have a different number of levels, you have to define for which bay you want to access the levels.

Storage.Bay CurrentBay = Rack.bays[<Index of the bay>];
int LevelNum = CurrentBay.levels.length;

In the same way, the slots are accessed through each level.

Storage.Level CurrentLevel = CurrentBay.levels[<index of the level>];
int SlotNum = CurrentLevel.slots.length;

For an example of how this is used, you can look at the code of the default options for the "Slot Assignment Strategy" of a rack or floor storage.