A good starting point is the motherboard memory QVL
here.
Suggest using a dual channel memory kit to ensure maximum compatibility and maximum memory performance.
Depending on what is required for this system, barest minimum would be a 2 x 4 GB dual channel kit. But a 2 x 8 GB dual channel kit is maybe the better choice as this allows more versatility to potentially run more programs faster/smoother, especially those programs that make use of lots of memory (graphics, multimedia, CAD etc).