I have an exchange server 2016 with mailbox server role already configured and we are trying to achieve high availability. In configuring the mailbox for H.A, how many mailbox server is required for on-prem HA config and for hybrid configuration ?
For high availability, at least two mailbox servers are needed. Details see: Deploying high availability and site resilience
Microsoft Exchange Server 2016 uses the concept known as incremental deployment for both high availability and site resilience. You simply install two or more Exchange 2016 Mailbox servers as stand-alone servers, and then incrementally configure them and mailbox databases for high availability and site resilience, as needed.