Another article is claiming it’s up to 44 million…

"Instagram could be a primary driver of Threads’ adoption. This is due to a badge assigned underneath your Instagram profile picture with a number. This number denotes your user number on Threads. These are thought to be chronological, with Meta founder Mark Zuckerberg holding the coveted “1” badge.

Dexerto has observed that the user numbers have now surpassed 44 million at the time of writing, and it’s likely that as more regions find that the app has launched, this number will continue to rise."