The first one is pass/fail/warning/done and determines how the event will be reported.
The second one is, essentially, the title of the event. It's what you will see in the Test Log in the left pane (the tree view of all the events logged to the test results) and also the title of the event if you select it and view the details in the right panel.
The third parameter is the detailed statement you can construct using VBS to say anything you want (technically you can do this with the second parameter too, but it often ends up making the title too long or complicated).
It isn't usually necessary to specifically convert a variable to a string in order to concatenate it in VBS since the variables are almost always of type variant.
x = 8 'this is the number of files in your folder, I'm sure you derive it more cleverly than this
Reporter.Report Event micWarning, "Calculated Number of Files in Folder", "The folder contains " & x & " files."
Something like that is usually all you need to do.
"The last 10% of any software project will take 90% of the budgeted time. The first 90% will take the other 90%"