I currently have 10-5TB hard drives in addition to my 2-5TB parity drives.  My question sounds morbid, but is not intended to be.   Let's say my house burns down to the ground and I lose all 12 hard drives, but kept the most recent copy of the parity drive backed up online.   Will that parity drive back-up be enough to rebuild all 10-5TB drives?  Should I back-up both parity drives, or will one be sufficient?  Any pointers for me?

No - parity does not work like that (you should read the description on the LimeTech suite to get a better grasp of how it works).    

 

Recovery requires enough good data drives in conjunction with the parity drive(s) for recovery.     If you have single parity then all the data drives but one need to be good to recover a failed data drive.   With dual parity then all the data drives but 2 need to be good to recover 2 failed data drives.

If you think about it for a minute, it should be obvious that parity alone cannot possibly have the capacity to be a backup of all the other drives.

 

Here is a good explanation from the wiki:

 

https://wiki.lime-technology.com/UnRAID_6/Overview#Parity-Protected_Array

 

And speaking of backups, you absolutely must have a backup of any important and irreplaceable files. Even if your house doesn't burn down, there are a lot of ways to lose files that parity cannot save you from.