You must decide which factors -assurance against risk of data loss due to backend LUN unavailability, optimizing I/O performance. The other two factors - optimizing storage space and speed of RAID rebuild don't apply here.
Minimum? 1 LUN in one raidgroup in Raid0
Max? Theoretically you can have a very large number of raid groups with up to 28 LUNS per raidgroup however how large a failure domain (single aggregate) do you want to create?