Batteries with N series cells like 6 or 12 for lead acid 12 or 24V are made with tight tolerances per cell, yet different cells may not match as well so balancing extends the life of series banks by equalizing the voltage. Thus VI=P bypass capacity to enable equal voltage on each under load charge or discharge.
6 Li-Ion cells =6S is a standard "24V" module for BMS then x number of parallel is selected for the Ah required load. Cascading each xP6S module then can managed digitally by a Master BMS up to any voltage usually <1kV depending on the kW load to minimize conduction losses and cost of copper such that the ratio of source ESR / load R resistance is a relatively small ratio (like 1%) to minimize load regulation error on the battery voltage and Dissipation Factor in heat rise of the cells.
The resulting battery array for BMS is then (xP6S)x N modules. The Dissipation Factor DF may be expressed as a power loss and heat rise of the cells just as it is done for capacitors. However battery cooling is essential and the DF of load must be chosen by design with thermal resistance and max Temp. rise of cells.
( confirmed by consult today with son-in-law U of T Prof in EE Power Eng.)
Tesla S and BMW 3i both use a Siliconix IC for dual port bi-directional capacitive RF coupling of data from the HV BMS side to the LV Master BMS for SoC , V matching for each module.
The weakest cell (Ah) discharges and charges the fastest will need the most bypass to prevent over /under charge voltage. The parallel banks don't have this problem.
The optimum number of series cells in a string is a question of BMS bypass capacity per cell, deviation of capacity in the string, and temperature and reliability margin. Also the time available for balancing (if not 24/7).
It is unanswerable without specs for the actual BMS design of bypass capacity from a cost limit per cell and the deviation is based on the supplier specs of mismatch from new to the life of the module and the max temperature rise of the batteries due to BMS and operational use.
Thus, the cost of quality batteries and deviation of capacity are underdefined variables that must satisfy the limitations of the cell monitor and balancer.
These are not trivial problems when you consider some BMS systems support up to 256 cells in series for 1kV.
Here is a patent on one such system. and its website
The above appears to be a passive bypass method. Another I know uses a half bridge DC-DC to transfer excess energy with LC storage to the other cells in series, covered by a patent, which is not handy at this time.