RAM: Crucial
CT2K8G4DFS8266

Description: 16GB (2x 8GB) 2666 MT/S 288-pin DDR4 SDRAM UDIMM (PC4-21300), Single Ranked x8, 1024M x 64, non-ECC, 1.2V
Standards: DDR4, UDIMM (PC4-21300)

This is the standard RAM used in a Coffee Lake motherboard. If you do a motherboard lookup at the Crucial website they then list this RAM as the one that they guarrantee works. It is based on the 14nm technology that yields tremendous performance from significantly lower power requirements.

The part number contains a "2K" in positions 3 and 4, remove those to buy just one.

This is Single Rank memory. Single Rank gets its job done in one pass through its chips, while Dual Rank requires two passes. Generally, you'll only want Dual Rank in special circumstances or on odd motherboards. Single Rank will give you better performance in nearly all scenarios, and is the one to buy when you don't know what to do. It usually costs about $3 more. OEMPCWORLD explains this more indepth. There are also times when only Dual Rank is available. They both work.