spares are global - not dedicated to a pool  rule is 1 spare for 30 total drives  for more info see the VNX white paper
i have 25 disks 20 used and 1 spare and 4 system  i will extend pool space and add new disks in same pool i need to add many disks how many disks will used as spare ? and  i will create another  new pool it will create new spare disk or no ??? last Q ... i create 3 datastore did all datastores share same spare disk or create there own spare disks ????
extend pool add new disks and create new pool ,change spare policy