Hi Yaron,
As best practice you can have MAX 16 dimension (13 custom + 3 standard ). As far as I know there is no limit to have max number of dimension. In principle if you increase the number of dimension it will impact the performance as well as its difficult to use the the system in perspective of end user because they have to make lot more selections.
On the other hand, you are planing to go with BPC on HANA , there should not be any problem but again its depends on how many dimensions you have.
For the benefits of HANA kindly go through the below links..
BPC on HANA - true innovation without disruption
SAP BPC on HANA | Bluefin Solutions
Shrikant