Hi,
There isn't a way to do this OOTB but you can achieve the same with some JavaScript.
Add a Content Editor Web Part below the List View Web Part on the page including your custom grouped view. Then add a javascript code block and search for the number of instances of the TD element with the style of "ms-gb". Do a count of these and that is the number of distinct grouped items. Output the result however you choose via the javascript.
Paul Leigh
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Software Architect and a
QuikR Microlight pilot in my spare time.