Atleast 3,815,207 users. (who have performed some meaningful GitHub action)

Via BigQuery:

   SELECT COUNT(DISTINCT actor) FROM [githubarchive:github.timeline];

The mosaic effect they used doesn't appear to be real, as no number has a horizontal gap between two parts.

However, GitHub user IDs are/always used to be sequential, create a new account and see what its user ID is. It's around 8.6 million at the moment.