It's extremely hard to get an accurate count of connected devices, so we're all just estimating. There's lots of sources giving rough values for things like smartphones or IoT devices, there's a reasonably common estimate of 5.6 billion connected Internet users, but it's largely guesswork for connected devices per user.

It's improbable that I'm off by an order of magnitude: 7 billion is far too low (we have 7.5 billion smartphones in the world!) and 700 billion is far too high; how low an estimate could we make without being unreasonably optimistic? 40b seems quite low to me - 7.5b smartphones, 5.6b connected users, 20b IoT devices, and commercial use of IPs - but if we took that value we'd be sitting at saturation for 36 bits of address space (60% utilisation is pretty darn good!) and the next decade would kind of suck.