We are creating a navigation with many pages and therefore need many subcategories. However 16 seems to be the maximum number of subcategories (Category > Subcategory). Each of those subcategories has a maximum of 10 subsubcategories (Category > Subcategory >Subsubcategory).
We do not have such restrictions. Please disable third-party plugins and customizations (if there are any) and check if this issue persists. If you use a caching plugin, make sure that caching is disabled for logged-in users. If this issue exists, please provide more details (e.g., your actions step by step with screenshots, etc.). This will help us reproduce and resolve the issue faster.
Seems to be a problem with adding pages through the customization > header > navigation > add new. After publishing and reloading it worked.
With many navigation points they seem to be cut off at the footer however. I made a new installation demo to show it to you: My Blog – My WordPress Blog (check pages > dropdown > third item)
If you do not see all 50 menu items in the header, please note that this is a limitation on the part of WordPress. Also, we do not recommend adding so many items as it affects the UI\UX.