They quite possibly may have changed it at some point, but claim up to 25 heating devices + 25 separate A/C controls (should you have AC fitted/controlled):
I can't speak for Evohome but agree with your comments about Tado. They aren't the most customer responsive, feedback engaged IoT company out there, but the products are generally well built/specified and the user interface is very good. There are some annoying things, like the inability to have a bridge repeater/extender and for me at least the fact the bridge only connects to wired Ethernet and half-duplex 10BaseT (which is blimming ancient and becoming a compatibility liability with some of the newest network switches which wont go down that low...) but there are workarounds and will be fine for 98% of folks.
In general I would still recommend them for CH and HW domestic IoT-enabled system.