Someone may have a script or you could export to PDF then into Word if it’s a long document and the italics is spread randomly throughout the text. Otherwise, you could search and highlight or find by character or font style if applied correctly and use the info dialogue. I’m so curious as to why you need the italics character count!
You may do more runs of that GREP Find/Change action, because there are fonts where the "Italic" style is not named "Italic", but "Oblique". Or because you are also want to find text that is formatted in Bold-Italic or other variants.