My sky hub became permanently rebooting at 16 devices.
My virginmedia hub is fine with 30+ connected.
My house has 4 smart tvs online, 3 sky boxes online, 2 apple TVs and 2 wireless printers. those things are online pretty much 24/7 but not in use all the time of course.
The BT Home Hub 5 also works fine with 25+ devices connected.
The only lacking router from major ISPs is the Sky Hub as it's single band, the HomeHub, TalkTalk router and Virgin superhub are all AC wireless and I believe they will work fine on way over 20 devices as the connection is split between both bands.