Is there a file manager that can serially number files and sub-folders in the currently viewed directory?
Ranger can do that. Ranger is a VIM-inspired file manager for the console available in the universe repository of all currently supported versions of Ubuntu. The GitHub page is here.
This answer is not meant to describe how to use Ranger but specifically to indicate that numbering files and folders is possible using Ranger.
Users can edit ~/.config/ranger/rc.conf to change the default setting of
# Disable displaying line numbers in main column set line_numbers false
# Disable displaying line numbers in main column set line_numbers absolute
set line_numbers absolute, a user can jump to specific a entry in the main (middle) by typing
N is the line number to the left of the file or folder name.
The following images, of Ranger in Ubuntu 19.04, shows line numbering in effect in the main (middle) column for the home folder and for /etc.