Since yesterday (May 15, 2019) SAP has released usage of HANA ScaleOut for Power9 (max 16 nodes in a ScaleOut Cluster)
BW and BW/4: 16 nodes x 16 TB = 256 TB max cluster size
S/4: 4 nodes x 24 TB = 96 TB max cluster size (min node size 6TB or higher)
Details: SAP Note 2055470 V75