Main memory can contain approximately 1800 objects, which you can distribute among the scan lists any way you want. You could put 1800 objects in a single scan list or you could put 90 objects in each of the 20 regular scan lists.
Scan lists are not fixed locations, they are virtual locations.
You can assign a given object to one scan list or to many.
There are 20 regular scan lists plus 2 special scan lists that cannot be scanned in combination with any other scan lists (if the 'Favorites' list is on, then the other 21 lists are off).
Now, consider V-folders: A V-folder is a place to store a backup of main memory. Including the 21 v-folders, the total number of objects that can be stored is 22 x 1800, which is 39,600 objects. But only 1800 objects can be in main memory at any given time.