You can put as much parity as you want it depends on your level of comfort with fault tolerance. If you use 1 parity drive, then you are safe against 1 failure. If 2 drive die at the same time,well your screwed. Your parity drive has to always be as large as large or larger than the biggest drive in your pool. But at the end of the day the number of parity you put comes down to how paranoid you are about protection