How to select every x files

@Alderyos - Go back to the thread I pointed out and look at the very first post. Download the SelectEx script and button:

v.0.5.8 - 2021.03.07 - fix ftp path handling
Command.Generic_SelectEx.js.txt (158.4 KB)

Menu-Button & Preview:
SelectEx_MenuButton_v0.2.dcf (2022.01.30) (39.7 KB)

Add the menu-button to a toolbar, then look at the "Select N-th.." button. Here's the code:

SelectEx EXACT MAKEVISIBLE NTH={dlgstring|Enter number to use for n-th selection:|3} 

It does exactly what you want.

If you are unfamiliar with buttons and scripts, reference this post: