https://forums.truenas.com/t/zfs-storage-pool-layout
https://forums.truenas.com/t/assessing-the-potential-for-data-loss
Those guides should cover basis and more.
Also:
Imagine that during the replacement of the failed disks, also the other fails 
and there are a lot of more things that can go wrong in the wrong moment… That’s why you should consider a backup strategy.