What is the required number of users to work in site resilient environment ?

Exchange Server 2013 and later uses DAGs and mailbox database copies (along with other features such as single item recovery, retention policies, and lagged database copies) to provide high availability, site resilience, and Exchange native data protection. The high availability platform, Exchange Information Store and Extensible Storage Engine (ESE) have all been enhanced since Exchange 2010 to provide availability and less complex management, and to reduce costs.