What is internal battery dc voltage and how do I calculate some battery specs?
It can be confusing when a product contains a battery and a voltage-changing inverter, as it's not always obvious which specs apply to the battery, and which the terminals.

This product has an internal nominally 50v battery, and a bi-directional inverter, through which charge and discharge takes place. It's designed to interface with rooftop panels, which typically have system outputs in the 350-550v range, allowing low loss connection through small wires from remote roofs. 

The maximum charge power is 5kW. This is the only charging limit specified. This is consistent with the charge being limited by a battery current, yet delivered through voltage interface at 350-550v. 5kW at a 50v battery happens to be 100A. However, if the panels were outputting 500v, this would be 10A at the powerwall/panel interface. As this product is supposed to be easy for homeowners to install, I think we can assume that its internal controls take care of charge management, when hooked up to a set of roof panels, and that we 'don't need to know' what's happening at the battery level.

Similarly, the calculation for battery Ah looks plausible (I've not checked it for accuracy or even order of magnitude), but again it's irrelevant for the user, they interface with the 13.5kWh energy capacity specification at the 500v terminals. I doubt that the battery is 50v, this is a nominal, lithium batteries have a wide voltage range over their charge/discharge cycle.

The 14.3A and 20A current specifications refer to the high voltage terminals, not the battery.

I read the data as saying 'stack up to 9' units regardless of whether their AC or DC. The limitation might be purely software/communication hardware, like how many ports the controllers have. It might be only one unit connects to the house and all the others pass-thru, so there's a current limit on some connections. It might be that to stack them they go through a combining unit, with a current limitation. It might be they all connect in parallel to the house so there is no current limitation, but there might be control stability issues, connect too many in parallel and one might fight the others. They might have qualification for only a certain energy density in domestic premises, without needing higher levels of fire-proof construction round them. There is so much software, design and regulatory stuff in these products that physics is but one minor limitation.