@Daniel_360 has one good method which is to change backgrounds (containers of loci) every nth locus. If you can rebuild your list that was, it would be the best choice.
Another would be to add in a marker image of your choice on every nth locus. That was the recommendation of the author of Rhetorica ad Herennium. He used a golden hand every 5th and a person containing the word “ten” in their name, like Tennessee Williams for every 10th image. But random access to the 50th item wouldn’t be that easy. You’d still have to scan through all the items. It would be faster looking for that marker image. My least favorite.
Still another method came up recently from @joereddington that I think @zvuv is referring to, called a skip list. Was that the idea you had @zvuv?
You would have to associate every nth location with a subject, an animal or a person and then remember that list as a table of contents to your first list. This is great if you have a pre-built list and don’t want to make major renovations on it.