See pp. 52-57. Memory is DDR4-2400 SO-DIMM which downclocks to -2133. Stated max is a pair of 8 gig modules for 16 total but it will run a pair of 16 gig modules for 32 total.
You likely will not get better performance with 32 gigs over 16. So you need to add one 8 gig module. You can, but I advise against doing it, add a 16 gig to the 8 gig to have 24. Because you would not have dual channel when mixing modules of different size, 24 gigs may actually perform worse than 16.
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