@Jidanni: Wrong task, see the topic of this one

Well I took a quick look in
https://en.wiktionary.org/wiki/Special:SpecialPages
and clicked on several of them.

All the lists I saw were numbered lists.

It's only natural when looking at a long list of items to want them numbered.

And one can even have up to 500 items shown in the list I was complaining about.

It's only natural when looking at a long list of items to want them numbered.

Not if you do not use the number for anything

1. Browse https://commons.wikimedia.org/wiki/Special:ListFiles?limit=500&user=Jidanni&ilshowall=1
2. Scroll down to the middle of the file.

How can you tell if you are looking at the 249th of 253rd picture?
Sure, if it doesn't matter to you then ignore the numbers.
But if it does, then one cannot live without them.
At least there should be a button to switch them on.
Or if numbers are bad, then take them off all those Special Pages.

The numbers could be within a current table cell. No need to make an extra column.

How can you tell if you are looking at the 249th of 253rd picture?

Which problem is solved if you know that you are looking at the 249th of 253rd picture? (Use case. See above.)

Or if numbers are bad, then take them off all those Special Pages.

If other Special Pages list numbers and consistency is your argument, then please share such arguments so we all save some discussion time. Thanks.

https://www.google.com/search?q=why+do+books+have+page+numbers

Page number is the only index for pages in books; whereas in the file list, each file is indexed per its file name. You may also refer to a given file thanks to its date.
To describe a use case, a story is often better; e.g. “While reviewing all files from a specific user opening them in new browser tabs, I would like them to be numbered, so I can easier remember the last file I have open.”
You can’t say “sometimes it is useful to have numbers”, we need to explicit “sometimes” with a precise story.

Adding a new column would enlarge that table which is already (too?) large.
Adding the number inside another column seems me confusing, I don’t know where it can actually state.

I think that kind of optional feature should be supported through a user script. You can create it with something like that:

var listFileTable = $( 'table.listfiles' );

listFileTable.children('thead').children().prepend( "<th>#</th>" );

var rows = listFileTable.children('tbody').children();

rows.each( function( index ) {
	$( this ).prepend( '<td>' + (index+1) + '</td>' )
} );