- Array capacity. Whatever your capacity requirements are, these are underestimated, most likely by the factor of two.
- Budget limits.
- 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.
- Expected lifetime. 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.