For HyperScale setup when doing Node expansion you will need to expand by the same amount of Nodes in the original configuration
I would recommend to start with a 3 Node Gluster and then expand with the first 3 Nodes, once this is successful then expand with the remaining 3 Nodes
By default a global DDB with 6 partition will be created and will be 2 DDB partition per Node
After the first 3 Node expansion the DDB will automatically do a DDB move Partition and distribute the DDB evenly across the 6 nodes
The final 3 Nodes will just be data Node and will not house any DDB.