Ok, first let me do some math. 18 x 2 =36, so where are your spares?
Also, how big are the disk, are the same type of disk,
Even though SAS disks can go as high as 28, i personally limit it a little, i've done 24's, 22's, 20's, 18's, 16's, depends on the environment, and how much risk you want to take.
Also, i would expand the rg and reallocation accordingly, (read the reallocate tr for best practices)