When you are about to build a RAID, make sure you understand your storage requirements.
The following points are significant:
Whatever your capacity requirements are, these are underestimated, most likely by the factor of two.
Expected activity profile, especially read-to-write ratio.
If there are mostly reads and few writes, then RAID 5 or RAID 6 would be OK.
If significant random write activity is expected, consider RAID 10 instead.
Whatever the projected lifetime of the storage system is, it is underestimated.
For a quick estimation of capacities for various RAID levels, check the online RAID calculator.