You can refer below links for memory configuration details for R720

https://www.dell.com/support/manuals/en-us/poweredge-r720/720720xdom/system-memory?guid=guid-7550b0f0-b658-4f09-a3f8-668d9ced36ae&lang=en-us

https://www.dell.com/support/manuals/en-us/poweredge-r720/720720xdom/general-memory-module-installation-guidelines?guid=guid-0f97a40c-2a63-4dc8-a8b2-607006d75804&lang=en-us

https://www.dell.com/support/manuals/en-us/poweredge-r720/720720xdom/sample-memory-configurations?guid=guid-afe58651-3836-4641-8d7b-463afac05ea6&lang=en-us