You would think this would be an easy calculation.
What makes it difficult is that every variety of grapes has different cluster sizes. And the actual grape size is different across varieties. And the size of the grapes and the clusters vary across vintages.
If you Google it, the answer varies from 75-240 grapes: quite a range!
And every year it is different. Paul goes through the rather laborious task of counting the grapes for each of our varieties (15 in all). He randomly selects clusters and then counts all the berries (yep, that is what they are called) in each cluster. It can take days, but then he gets a fairly good idea of how many tons of grapes we will be harvesting in September and October (and maybe even November).