I think there are other, more important considerations in determining how many parity drives to use with snapshot RAID (as compared to considering number of data drives).
1) Do you frequently delete or modify files on your data drives? If so, dual-parity or better is recommended to increase your chances of being able to recover data if a drive fails before you update the parity.
2) Are most of your drives 2TB or larger? If so, at least dual-parity is recommended, since it takes many hours to restore the data from a failed drive, and during that time all of your data drives and parity drives are undergoing the stress of being heavily accessed, which makes a drive failure during the restore more likely.
Only if you answer 'no' to both of those questions should you then consider whether you have too many data drives for single parity.