I think this is a relevant question.  I was reviewing documentation:

https://knowledge.domo.com/Visualize/Managing_Cards_and_Pages/01Managing_Pages

In the "Moving pages in the hierarchy" section on this page it states:

"When you move pages, you are given the option to assign them as top-level pages or subpages. When you choose to assign pages as subpages, you are asked to select their new parent pages. You can select any top-level or second-level page as a parent page. (You cannot select a third-level page as a parent page because there are only three levels in the page hierarchy in Domo.)"

I tried and don't seem to have luck assigning a page to a subpage.

So it appears that the hierarchy is only 2 levels deep

1. Top page

2. Subpage

3. I haven't had luck creating a subpage at this level