Yes that is true. The Pro-106, Pro-197 and the GRE equivalents PSR-500 and PSR-600 can only have 22 scanlists (20 programmable +1 favorites list + 1 Skywarn list). And no there is no way to increase the memory.
However, you could make use of the 21 Virtual Scanner Memories (V-Scanner folder). You could program each V-scanner folder with 22 scanlists. You could only load one V-Scanner folder into working memory at a time, but you could easily switch V-Scanner folders and listen to a different set of scanlists.
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.
Being able to assign a scanlist number to each individual object means you can create a single scanlist with your favorite conventional analog and digital frequencies, along with any selected TGIDs from analog and digital trunked systems. Just think of the possibilities!