When you deploy multiple Exchange server in your environment, you can distribute same digital certificate across all exchange servers. There is no need to buy or issue different certificate for each Exchange servers.
When you first purchase and install a certificate on an Exchange server, you can export that certificate and import it on other Exchange servers as well. Its worth mentioning that You can also export certificates from Exchange 2007/2010/2013 and import it in Exchange 2016.
This guide will walk you through the steps to export and import certificate in Exchange 2016.