I host the site with that calculator. It can be confusing:
1) the 8 bay units (p4300 models) are mode from a single RAID 5 array
2) the 12 bay units (p4500 models) are 2 RAID5 that are striped (RAID 5 + 0) or RAID 50
What my calculator is showing is simply the HARDWARE RAID of the individual storage modules.
If you have more than 1 module in your cluster you have the option of doing Network RAID 10 (network striped, 2-way replication) that will consume 2x capacity, but allow a single node failure without your storage going offline. The network RAID level can be configured on a per-volume basis, so you can run higher redundancy for more important volumes, or less for archival etc.
Capacity and throughput scale with the number (and type <7.2k MDL vs 15k SAS>) of drives and number of modules.
The total number of spindles does NOT have a practical limit, as you can continue to add modules to your cluster to increase througput.
The 16TB P4300 you are refering to is the Starter MDL SAN solution.
It is made up of (2) chassis, each with 8x1TB drives. If you use Hardware RAID 5 on each module, after formatting and overhead you have a total of 12.48TB usable of Network RAID-0
or 6.24TB of Network RAID-10 (or any combination of the 2, since the Network RAID level is configurable on a per-volume basis)