How many cells are in one granule of instant yeast? I'm purely curious and google has failed me.
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.