MICROSOFT ExchgStdCAL 2019 SNGL OLP NL DvcCAL (min 5)

Client access licenses (CALs)

a CAL is required for each user or device that accesses the server software.
Standard: designed to help users be more productive from virtually any platform, browser, or mobile device, with features in Exchange Server 2019 that help your users be productive no matter where they are—while helping protect your organization’s data. To enable Standard CAL features for a user, the user must be licensed with the Standard CAL.