for sata 2tb disk it should be around 1.64tb per data disks. so with raiddp 2x11rg you should get 18x1.64=29.5tb. One rg with 18 data disks would be same size, but you only have 2 parity disks instead of 4. But you have 4 spares 🙂
With an update of another 24 disk you could go for 3x15rg for example with only 3 spares.
If you have a spare ssd storage pool partition you should think about adding one to your sata aggregate as they really help with io bottlenecks on sata drives.