Hi,
How many SAS disks you have ? You don't need to create spare. Let say if you have 25 SAS disks available and you created aggr of 24 disks remaining one disk would act as spare.
and correct command to create aggr with 24 disks and 64 bit is:-
aggr create "aggr name" -B 64 -r "Raid group Size (24 in your case)" "Number of disks {24 in your case}.
Hope it will help.