How I go about this is by creating a calculated field which I then show or hide based on a count or sum of n in the filter. I believe titles are static and show even when no data is present,
Your filter to determine whether to show or hide the message, this one is set to show if less than 10 customers are in the view.
With a single order date selected the message shows up.
With all selected the message disappears.
You can take a similar approach with whatever n is and apply whatever dimensional filters you need. Let me know if you have any questions.
Thanks so much, Ivan! That works perfectly and is beautiful in its simplicity.