If you plan on building RAID 5 with a total capacity of more than 10 TB, consider RAID 6 instead.
The problem with RAID 5 is that once a member disk has failed, one more failure would be irrecoverable.
To reduce the probability of double failure you need to monitor the array and/or use hot spares.
However, even if the RAID rebuild is commenced immediately once the drive fails,
it is required to read the entire array in order to complete the rebuild.
Although the probability of encountering a read error in any particular read operation is very low,
the chance of a read error occurring increases with the array size.
It has been widely speculated that the probability of encountering a read error during the rebuild
becomes practically significant as the array size approaches 10TB.
Although the speculation relies on certain assumptions which are not likely true
(refer to the bonus section for details), having to restore from the backup is not pleasant.
By the way you do have backups, do you?
RAID 6, being capable of correcting two simultaneous read errors, does not have this problem.