Each differential other than the first one has one input already taken, as it's connected to the previous differential (the second differential is connected to the first one, the third to the second one, and so on). Thus we are left with only one free input, and we can add only one motor for each differential. Unfortunately, the resulting high torque makes chaining adders in this way fairly risky.
when more than two motors need to be coupled, it's usually a better choice to use motors of the same type and hard-couple them. This is true not only because it takes less space but also because with more motors, there is more torque to transfer, and differentials are not fit for handling high torque. Hard-coupling with knobs (shown in Figure 18-10) is a reasonable alternative.