64 disks is the maximum size of a drive partnership group inside the dynamic pool.  In order to expand the pool you need to create a new DPG, which requires a full stripe width of disks, plus a spare.  So if your current stripe width in the dynamic pool is 12+1 for example, you will need to add 14 additional drives to the pool in order to expand it beyond 64 disks.
Add 6 new disk to Unity 650F have only one pool of all flash however the expand pool is disabled, currently there are 64 disk in pool, current running version is 4.3.1.1525703027, can anyone help with the issue.  Why the Expand option is disabled? is there a Max Number issue in the pool ? 
Add 6 new disk to Unity 650F, Unable to Expand Pool