CAS Array and SSL License

Hi All,

I want to buy Commercial SSL Certificate License for My Exchange Server Environment. 

My Exchange Enviroment is :

2 CAS Server (mail1.domain.com and mail2.domain.com) 

2 Mailbox Server (mbx1.domain.com and mbx2.domain.com)

I'm Using CAS Array for my 2 CAS Servers and DAG for my 2 Mailbox servers. 

I confuse how many SSL Certificate License should buy.

Regards

Rengga Patria