I think there is more to it.
It is complicated to tell exactly how many users are there, because (at least I think) numbers shown at the top of https://data.syncthing.net/ are for devices, not for users although it says in description. And for syncthing to actually work as intended, you need at least 2 devices for one user (for example I use 6).
But count users based on number of devices is also non trivial task not only because “Number of Devices in Cluster” in 50% column isn’t average but median.
But we can at least try to count devices:
I think best way is to use number “Connections” from http://relays.syncthing.net/. Instead of “number of devices with reporting enabled which showed in last 24 hours” it shows “number of devices with ‘Relaying, default relays’ running right now” (As far as I know one device can be connected only to one relay at the same time.). From data usage we know that 94% of users which are reporting are using “Relaying, default relays”, for those with reporting off I expect this number to be lower. Considering only small part of those devices runs 24/7 I dare to guess there is at least 3 times more devices running regularly.
And as @jerryjacobs mentioned there is number of devices running “off the grid” with their own relays, relay pools and discovery servers. And that number is next to impossible to guess right.
But ultimately @calmh is most qualified to tell how many devices there is from main discovery servers (or at least he could be if he wanted I guess, I don’t know if discovery servers actually are collecting this info). But “off the grid” devices eludes him as well.