Currently, the maximum is eleven volumes (root and ten data).  That means you can add up to six additional volumes to your VM if needed. A volume can be from 20 GB to 1 TB in size.  If your application requires volumes larger than 1 TB, you can aggregate volumes to present a larger amount of contiguous space.

You can select different volume sizes by clicking on the up/down arrow on the "Disk Offering" line.

Note:  There is a limit of 11 volumes per virtual machine: 1 root disk and up to 10 data disks.  If your virtual machine has already reached its limit, you will be presented with a message like the following:

Click here for information about setting up RAID on Linux volumes.