Datawatch's marketing around automation can be a little hard to find, and even if you find it, a little confusing.
If you have three project files, then the easiest syntax is:
C:/Progams/Monarch/Monarch.exe "C:/Documents/Projects/Project1.dprj" /pxall
C:/Progams/Monarch/Monarch.exe "C:/Documents/Projects/Project2.dprj" /pxall
C:/Progams/Monarch/Monarch.exe "C:/Documents/Projects/Project3.dprj" /pxall
I can't find the backslash on my keyboard, but assume you'll work out how to fix that.
However, since v11.5, automation is not enabled by default but costs extra, and since v13. something a few months ago, is apparently replaced by a desktop installation of the Automator server product.