Can anyone suggest how many hot spares are required for a RAID 6 array?

Current intention is to configure as a DAS with with 2 Disk Groups - RAID 6 for SAS Drives and RAID 6 for SATA Drives with the following provisions:
1 X MD3000i - 15 X SAS
1 X MD1000 - 15 X SAS
1 X MD1000 - 15 X SATA

TIA

I do not believe there is a requirement to have a hot spare, but with that being said I would have designate at least one hot spare per disk group.

Also, note the following from the support matrix:
http://support.dell.com/support/edocs/systems/md3000i/en/SUPPORTMATRIX/Sup_Matx.pdf
SAS and SATA disks cannot be mixed in a disk group.
A hot spare for a disk group must be a physical disk of equal or greater size than any of the member disks.
The hot spare for a disk group with SAS physical disks must be a SAS disk. (and same for the SATA)

You of course should have time to replace a single failed drive with RAID 6 but I always try to put a hot spare per enclosure if I can and then just mark the replaced drive as a HS for the enclosure when it is replaced. This of course works best if all the drives are the same size in a given enclosure.

One other thing to consider is having two HS if your drives are 500GB or larger as the rebuild time might exceed your comfort level on the bigger drives.

I would think that 1 HS per enclosure would be fine, even with 2 TB drives. With RAID6, there are two parity drives, so a RAID6 group with 1 failed drive has the same level of protection as a healthy RAID5 group. You would certainly hope to have your array finish the rebuild before a 2nd drive fails, but that's not a disaster. Disaster if when a 3rd drive fails before the first drive rebuilds, but having additional hot spares isn't going to prevent that.