There is no limit to the ammount of cards allowed in a zone, unless the card says "zone exclusive". There can be only 1 zone exclusive card in a zone at a time. Pg 21 in the rules
Good on you for including the reference page.
As a point of interest, I
often find that I am playing with, like, 10 cards in a zone between my stuff an an opponent's stuff. It's to the point that I'm about to embark on a project to create a board to better accommodate our playstyle. An exact replica, but on a vinyl sheet I can roll up that measures in the neighborhood of 3' x 3.5'.
-nihil