Hi Michael,
When I want to find this out, I just look at the number of phyical drive slots. You could install the Cisco VSM RAID Viewer over your remote connection and use that, or you can use the CLI command documented in MegaCli Common Commands and Procedures. That command is as follows:
/opt/MegaRAID/MegaCli/MegaCli -PDList -aALL
A MSP-2RU has 12 slots and a 4RU has 24.
Hope that helps,
Sean