I'm playing WoW currently, on and off. it has 11 characters limit per server with 50 characters limit across all servers. Age of Conan I stopped playing only a couple months ago, and off the top of my head I remember having 8 characters on a single server (I had a monthly subscription), and I still had option to make more. I remember having exactly 9 characters on Warhammer. I don't remember however if I had space for more toons on that server though.
Another thing to concider with ESO, please do remember that it's a SINGLE Mega Server. You won't have any other servers to make more characters when you reach a limit on your initial server like in other games. So they'd BETTER give us sufficient amount of character slots. Enough to roll one class of each race (9 in ESO case), or enough to roll one of each class (if there are more classes than races) is a bare minimum in my book of standards.