There was no mention of an Arena anywhere.
And why would you want more than one anyway?
What if your already a Grand champion in one Arena you will only fight monsters once a Week
And then you join an another Arena To fight other Warriors And be the Grand Champion in that Arena
because i think your gonna get tired fighting monsters and then you keep waiting a week to fight a Monster
And each Arena have a different Raiment,. in Oblivion Kvatch there was also an Arena But the city got Destroyed by those Daedra's i know it from lore here is a link to kvatch before attack and you can see the Arena
http://www.uesp.net/wiki/File:Kvatch_FGIllustration.jpg
and if it didn't get Attacked we would have two arena's in Oblivion
but i think two of them is enough and it's just an idea not that i really want it