I think the MAC Address of the router is known too, so there's no duplicates.

I have to wonder whether the MACs are "globally" unique - what's stopping an OEM using the same MAC? Also, I've seen router firmware features that will autogenerate a MAC for a guest network.

That comment was meant to be humorous, not serious.

That's why there was federal malware that captured MAC addresses on tormail when it was shut down. Easy for them to get Google to match it up to an identity

Coupled with the rest of the backed up data, it doesn't really matter if the SSID were not unique.

Try tens of millions.