They've seen 3997 distinct devices - with the caveat that this number is inflated by quite a lot of people running custom ROMs, which are registered as separate, unique devices: 1363 devices appear only once, and according to the developers, this is because custom ROMs can overwrite the android.build.MODEL variable.
Looking deeper into the data, you'll see that Samsung is by far the most popular brand for Android, followed by HTC. Gingerbread is still the most popular Android version - Ice Cream Sandwich has just 8.5%. Some work to be done there. About 13 resolutions make up the bulk of the logged devices, with a slew of less popular ones dragging behind.
Infographics like this usually make me itch, but this one is interesting because the developers aren't complaining about this diversity, nor are they blindly praising it. "Fragmentation allows users to take their pick from thousands of devices. You can choose from phones with 3D screens, projectors, CDMA, GSM, or even CDMA & GSM. You may not care that Tag Heuer has made an Android phone but at least one person does (and they use OpenSignalMaps)," they write, "It's a triumph for Android that as a single OS it can target so many markets."
However, they're honest about the challenges this poses to Android developers. "The proliferation of devices with their associated screen sizes, internal hardware and custom ROMs creates some difficulties," they explain, "We spend a lot of time making the app presentable (or at less functional) on exotic devices - this is the most common request we get from app users."
And this is exactly right. Android's diversity is its strength, but like with so many things in life, it's also its weakness. I like that nobody is imposing a certain screen size or resolution on me, leaving me free to pick the optimal screen size for me (4.3" hits the sweet spot for me). I don't have to fiddle with a tiny 3.5" screen, nor am I stuck on a single resolution (seriously Microsoft?).
It's this diversity and choice that permeates every aspect of Android, and yes, that's problematic in many areas, but honestly, I personally wouldn't want it any other way - I'll take the downsides for granted. Be sure to check the data OpenSignalMaps has collected - it's quite fascinating.