I have seen specs with 16 GB but also with 32 GB.

From what I could tell by reviewing the service manual and the spare parts specifications, the HP Pavilion 13-s128nr x360 you could use either: "Support for 6.35-cm (2.5-in) hard drives in 7.2-mm (.28-in) and 7.0-mm (.28-in) thicknesses" "Support for M2 SATA-3 next generation form factor (NGFF) 128-MB triple-level cell (TLC) solid-state drive"