For a net new SSD tier supporting AO or volumes the minimum would be 6 x 400GB, optimal would be 8 x 400GB disks.
Although from a performance perspective if you only have 10TB raw in SAS you might be as well building the existing tier out, otherwise you'll have a very large performance imbalance between the two tiers.
Since it's a relatively small configuration you could also consider using fewer SSD's (2) and Adaptive Flash Cache and use some of the remaining budget to add to the SAS tier, that way you can always build the SSD tier out in the future to support AO as required.
You can also combine the two approaches AO and AFC but you'd need to deploy the larger of the two minimum (AO minimum).
Take a look at this it will help with AO and AFC.
https://www.hpe.com/h20195/v2/getpdf.aspx/4AA5-5731ENW.pdf?ver=3.0