I don't think that's really known, nor is it easily available information, given that you can use Zotero without an account and can certainly use it without syncing. There are, e.g. about 7.5 million Zotero accounts that have been created since it started, but it's highly unlikely that even half of those correspond to currently active users.

There's a 10 year old blog posts by Zotero's director going into the problems with various available metrics: https://quintessenceofham.org/2011/05/21/on-usage-figures/