7 Cell Limited Motor Run events obviously have a 7 cell limit. The Formel Pylon class has a 12 cell limit, etc. So now, you have fixed the voltage. For simplicity, we will assume that a Nicad puts out 1 volt per cell. At high currents it is less, and at low currents it is more.
Many tasks limit the weight of the battery pack, so the number of cells is up to the competitor.
The 'open' events have no restrictions. The FAI rules for F5B allow for a 1100 gram battery pack, with a 42 volt maximum. This has more to do with the choice of motor than the number of cells. Since duration is a function of current only, the only way to keep the power output constant is to raise the cell count with the current drop. As they say, Watts is Watts.