Best to use 2U machines with the maximum number of disks IMHO. A 2U machine should be able to have 5 disks.
I say: 9 Disks without problems. e.g. pcicase http://www.pcicase.de/catalog/produktweb/IPC-C2-X/IPC-C2D.htm
Best to use 2U machines with the maximum number of disks IMHO. A 2U machine should be able to have 5 disks.