How many cells are in one granule of instant yeast?
This article estimates 20 billion viable yeast cells per gram of dried yeast. A yeast package is about 7 grams total.

The granule size will vary from brand to brand. Even within a packet, granule size is not completely uniform. To get the estimate, you'd need to weigh out a sample (say 1g or 0.1g), count the number of granules in that sample, then do the math. The larger the sample you count, the better estimate you'll have.