The number of nodes in an HBase cluster might also be driven by required throughput for disk reads and writes. The throughput per node greatly depends on table cell size and data request patterns, as well as node and cluster configuration.
You can use YCSB tools to test the throughput of a single node or a cluster to determine if read/write throughput should drive the number of nodes in your HBase cluster. A typical throughput for write operations for one RegionServer is 5 through 15 MB/s. Unfortunately, there is no good estimate for read throughput, which varies greatly depending on physical data size, request patterns, and hit rate for the block cache.