As you may know, Microsoft recommends:
|Exchange 2019 Role
When we first learned about these memory recommendations, they seemed very high, especially when compared to Exchange 2016. These recommendations are appropriate for installations that have 1000+ mailboxes.
But what about installations that have fewer mailboxes? Does the server need this much memory? It depends. We've found that an Exchange Server with the Mailbox Role that has roughly ten to twenty mailboxes requires around 16GB of memory when running on Windows Server Core. We typically configure the Exchange Server with four vCPUs and get excellent performance from Outlook Web App and the Exchange Admin Center.
Of course, your experience may vary but unless you are placing a hefty load on the Exchange Server, you probably don't need 128GB of the Exchange 2019 Server for ten to twenty users.