I don’t know of any issue specifically related to the number of drives in the pool.  All else being equal, running with more drives does not give lower performance, and should generally enable more performance, It may just flatten out depending on whether your application can keep all your drives busy or not.   You should also consider the fact that you should not be expanding pool LUNs very frequently in small drive increments. When you add new drives to the pool, the new allocations for any LUN will come from those new drives till all drives the pool are equally utilized.
I read in the EMC Clariion Virtual provisioning pdf that the max allowed drive increments for a pool on a cx4-960 (aka ns960) is 180 disks. Is that a realistic pool size or is the reasonable pool size smaller, say 120 disks or 150 disks?  I currently have a 110 disk raid 5 pool set for our vm environment and I expand it in 5 drive increments but at what point should I consider creating a 2nd pool?  Any best practices or experiences found?  thanks.  Jim
NS960 disk pool size recommendation