Did you see this .pdf spec:

https://i.dell.com/sites/doccontent/shared-content/data-sheets/en/Documents/Precision-7720-Spec-Sheet.pdf

It seems to suggest you can install up to 32GB of "super-speed" 2667Mhz DDR4.

I wonder why the document limits it to 32GB, not 64GB.

If you have the XEON processors, yes it will support up to 64 ECC and Non-ECC. The intel I7 support up to 32 only.

The  i7 will support 64GB of of non ECC memory running at 2400MHz, and 32GB of non ECC running at 2667MHz.  The only real difference the Xeon gives you is ECC memory and slightly higher clock speeds.  Unless you need ECC, Xeon isn't a good deal.