The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.
Hi Ameen,
"one server or two server "
It depends on the number of resources and users, but you can start with one and upgrade your infrastructure. But, I advise you to start with a final infrastructure before making your migrations. This way, you can do the tests (example: high availability) before migrating your users.
"can i add the exchange servers 2019 on the DAG".
Good question :) I don't know whith Exchange 2013 but 2016 yes:
https://support.microsoft.com/fr-fr/help/4488079/exchange-server-2016-allows-adding-exchange-server-2019-mailbox-server
BUT, I prefer to start with clean installation with new DAG.
Best
jmb