It depends what you need from the DAG. You can have several types of DAG - a "normal" DAG and a "Lagged Copy DAG", which is used more for disaster recovery.
How many replicas you should have is (in my opinion) more of a business decision than an IT one.
A "Normal" DAG is basically a copy of specified mailbox databases. You would have multiple replicas when you want failover to occur transparently to your end users. This allows several Exchange servers do go down (for maintenance or otherwise) and keep your mailbox database online.
A "Lagged Copy DAG" is still a DAG which replicates your mailbox database, but in a slightly different way. You can set a lag period on a lagged DAG so the replica is effectively a copy of the main database at some point in the past (by default 14 days, IIRC). Once a transaction log file is finished (i.e it reaches 1MB and another is created) on your active database copy, it is immediately copied to all lagged replicas but is not replayed immediately. This transaction log will stay on the lagged replica until the lag period expires, at which point it is written to the lagged copy database.
With that information, you should be able to give management an idea of what Exchange can do with regards to high availability/disaster recovery and possibly recommend a solution, but let them ultimately decide.