Below threshold Windows Aero will show thumbnails. But over threshold, because the thumbnails would be too small, it’ll show list instead.
Set:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband\NumThumbnails
to DWORD(32bit) 10 or whatever number you want.
Reference: tenforums.com : Windows 10: Change Taskbar Thumbnail Threshold to Show List in Windows 10