You are on the right track.
The maximum number of 'disks' per cloud ONTAP aggregate has not changed from 6. There are several factors that go into that number, but mainly it's because AWS doesn't recommend any more than 6 'disks' (EBS Volumes) to be used in a RAID 0 configuration.
So, to address your questions for the upcoming release of Cloud Manager and Cloud ONTAP:
Aggregate Maximums:
- Aggregate Size: 6 disks max (both GP2 and Magnetic)
- Aggregate capacity:
- GP2: 6 x 8TB = 48TB RAW
- Mag: 6 x 1TB = 6TB RAW
Max Flexible volume size based on Max aggregate size:
- GP2: ~ 42 TB usable per aggregate
- Mag: ~ 5.2 TB usable per aggregate
The Flexvol capacities are aproximations, but also based on what I've seen. I believe the overheads you used in your calculations were a bit too high.
Hope this helps.