Here's a white paper on SN5428 which gives you workgroup test results with 500-2500 concurrent users. This might help you plan accordingly.
http://www.cisco.com/warp/customer/cc/pd/rt/5420/prodlit/pxdst_wp.htm
The MDS 9000 family is aimed at lowering the total cost of ownership and delivers intelligent network services such as multiprotocol/multitransport integration, Virtual SANs (VSANs), comprehensive security, advanced traffic management, sophisticated diagnostics and unified SAN management. You check the product documentation for details.