Metals are used in 1 direction for the very simple reason that if you route a horizontal layer vertically, it can block the path for the same metal routes.
| x----------------
| x--------------------
| x----------------
------------------|
I've tried to draw this, but if you see the vertical route of the metal layer , it is blocking the other horizontal routes on the same layer. Although it is not a strict rule, you can route in both directions when you are confident that it will not block or congest your design.
9 or 12 metal tracks mean that in the design process, you are allowed to use max of 9 or 12 metal layers for your connections.