if it's an integrated NS40, you define hot spares when you select disk templates.

http://corpusweb130.corp.emc.com/ns/common/post_install/Config_storage_integrated.htm

If it's a fibre enabled unit, you define them manually in Navisphere. EMC recommendation is 1 hot spare per 30 disks.

http://corpusweb130.corp.emc.com/ns/common/post_install/Config_storage_FC_NaviManager.htm