Do you really use version 5.21? That is an ancient version of UltraEdit. The oldest version archived by me is v10.10c released February 2004.

There are multiple configuration settings in UltraEdit v2025.2.0.8 which determine the behavior of UltraEdit on opening one or more files with a double click or the Explorer shell integration DLL of UltraEdit.

There are at Advanced - Settings or Configuration - File application - Miscellaneous the settings:

• Files selected in Explorer will open in a new single instance
• Allow multiple instances
  • Maintain separate process for each file opened from external application

The help with a description of these settings is opened on clicking on the button Help in the configuration window. The UltraEdit Wiki page Settings:Application layout:Miscellaneous explains these configuration settings, too.

Further, there is at Advanced - Settings or Configuration - File handling - Session options the setting:

• Open from Explorer also loads file list

There is also the button Help in this configuration window which on click opens the appropriate help page. The UltraEdit Wiki page Settings:File handling:Session options describes this setting, too.

The ancient UltraEdit version 5.21 has perhaps just the setting Allow multiple instances which is most likely checked resulting in starting a new instance of UltraEdit with just the double clicked file opened in this new, second UltraEdit instance. The before already running UE instance keeps running and has the 30 files still opened. Just switch to this first instance of UltraEdit. I suppose that you are using the combined view of applications for the Windows taskbar and see therefore just one icon for the two or more running UltraEdit instances. You have to click on this single icon and a popup menu appears for selecting next one of the two running instances if there are more than one UltraEdit instances running currently. It is of course also possible to reconfigure the Windows 10 taskbar behavior to always show separate icons for each running instance of an application or show them only combined on Windows taskbar is full. There can be pressed and hold the left Alt key and next pressed once or more times the TAB key to switch between windows of the running applications. The switch to a different application window is done by Windows on leaving the TAB and next also the Alt key.

It could be necessary with ancient UE version 5.21 to drag and drop the (selected) file(s) in Windows File Explorer on the UltraEdit application window to open them (all) in the already running instance of UltraEdit in addition to the already opened 30 files. That is a quite complex task to execute if either the Windows File Explorer on the UltraEdit or both application windows are displayed maximized. In this case it is necessary to click with the primary (usually left) pointing device (mouse) button on the (selected) file(s) in Windows File Explorer window and hold the button. Then drag the pointing device pointer over the symbol of UltraEdit icon in Windows taskbar and wait here with still hold button a second until Windows brings the UltraEdit application window to foreground, then move the pointing device pointer with still holding the (mouse) button over the now visible UltraEdit application window and leave there the (mouse) button.

See also the forum topic Quick open a file in running instance of UE with Total Commander. There is the very small third-party application ueFastStart.exe which can be used also with Windows File Explorer. The file association for files to open in UltraEdit must be modified to open the files with ueFastStart.exe instead of uedit32.exe. There is started on a double click on a file of which file extension is associated with ueFastStart.exe the executable ueFastStart.exe with the file name passed by Windows File Explorer as command line parameter to this application. This application now looks up in the list of top-level application windows if there is one with name UltraEdit or UltraEdit-32 in the application window title and sends the file name with a message to that application window like it is done by Windows on using the drag & drop method to open a file in an already running UltraEdit instance. If ueFastStart.exe cannot find an UltraEdit application window because of no UltraEdit is running, it looks up in the Windows registry under key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\uedit32.exe for the fully qualified file name of UltraEdit as added by the installer of UltraEdit during the installation (for all users) to the registry and starts now uedit32.exe with the double clicked file name as command line parameter as it is done by Windows File Explorer on having a file extension associated with UltraEdit. Then the very small program ueFastStart.exe terminates itself in both use cases.