Confused about number of CAS arrays....

Hi

We are in process of implementing exchange 2010 in our environment.

• Fact 1: We have over 10 AD sites
• Fact 2: Exchange 2010 CAS servers will be in ONLY 2 AD sites (headquarters & DR)

I am not sure how to go about my CAS array. 

1. Do I need 10 CAS arrays(1 in each of the 10 AD sites)?  This one is neither desirable, nor something that makes sense.  But you guys know better

OR

2. Do I only need 2 CAS arrays (1 in each of the 2 sites where exchange will reside)?

Thank you