file explorer's frequent folders quantity

None that I've seen and none that out tutorial guru @Brink is aware of either. About all you can do is pin the ones you use a to to Quick Access.

Hi, if you want more, you can try a 3rd party program such as Listary (free for personal use).
Amongst its functions it offers favourite and recent folders, expanding in a tree.

This applies not just to explorer but to any open/save dialogue box.

Simply middle click on an empty space.


Then there are the programs offered by Actual Window Manager which add buttons to the title bar (as you can see) of most programs/dialogues, including favourites, recent, on top and a lot more. Not free.