R820 have 48 DIMM slots, and you can use all 48 slots only If you have all 4 CPU populated. With 32GB RDIMM (Max size RDIMM supported in R820) maximum memory you get is 1.5TB. R820 also support 64GB LR DIMM. If you populate all 48 slots in R820 with 64GB LRDIMM you get 3TB memory. I am not 100% sure whether this is a supported configuration. It looks like a valid configuration from below link.

https://www.dell.com/support/manuals/en-us/poweredge-r820/r820systemownersmanual-v3/sample-memory-configurations?guid=guid-03b6bb5f-2cb0-4515-816b-605f06b7fa67&lang=en-us 

Hi,

Thank you for replying.

I think JGGRT is the part number of 32Gb RAM LRDIMM.

I cant find the part number of the 64Gb RAM LRDIMM. Do you have this information?

Thank you.

Hi, Max memory size -> 1.5TB, and please see below, I can only find  32GB from the official website.
https://dell.to/36Rb24t

· Up to 1.5TB of LRDIMM memory (48 x 32GB LRDIMMs)
· Up to 1.5TB of RDIMM memory (with 48 x 32GB RDIMMs)
· Up to 128GB of UDIMM memory (with 32 x 4GB UDIMMs)

And below is the part number you're looking for, but to make sure if this is compatible with R820, please note that it hasn't been tested out officially hence we can't give you an answer.

Part #:G2K02  / DIMM,64G,1600,8RX4,4G,DDR3,LR / 64GB, DIMM, 1600MHZ, 8GX72, Buffered, DDR3, 240 Pin, 

Thanks for your enquiries, have a good one!