Usually end up with about 4GB spare on most I've seen :S if you clean out some of the old system files you can increase it a bit though - I've managed to get the OS usage down to ~14GB that way though it tends to creep up again.
I stuck a 64GB SD card in mine to help with space.
Agree with this. Personally I'd go for more but if its just browsing/media it can work.
I had an Asus T100 with 32Gb eMMC, stripped out all the crap and recovery nonsense and it was more than useable.
We have 4 Asus T200's in the office, usually end up with 6-7gb free with win10 and office installed. Had to uninstall office from one of them when upgrading to win10 due to space. "My Documents" stayed on the tablet but "pictures/videos/music" we moved to a micro SD card as some of the users like to sync phones to them.