How to Pass MULTIPLE filenames to a Context Menu Shell Command?
You can use Send To for this. It supports multiple files.
In case this website goes offline:
Open shell:sendto with Windows + R or paste it into your explorer address bar. It should redirect you to:
C:\Users\<yourusername>\AppData\Roaming\Microsoft\Windows\SendTo
Create a shortcut to your program in this folder and you should see it in your explorer right-click menu under Send to
You may want to look at this post, as it says that this isn't really possible to pass multiple files to a single instance and you must rely on some form of IPC(Inter process Communication).