There is no simple programming solution for this. Although VBA has a 'sentences' collection, that has no correlation with grammatical sentences. For example, consider the following:
Mr. Smith spent $1,234.56 at Dr. John's Grocery Store, to buy: 10.25kg of potatoes; 10kg of avocados; and 15.1kg of Mrs. Green's Mt. Pleasant macadamia nuts.
For you and me, that would count as one sentence; for VBA it counts as 5 sentences.

You might be able to use Greg Maxey's 'Sentence Deducer' Addin (Deduced Sentences) to help work out where grammatical sentences end. See also his code in: https://www.msofficeforums.com/97131-post14.html

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]