I don't think it is the amount of drops taken, within reason, that makes people sick, it is the concentration of the free ClO2 in the dose.
The more drops, the more dilution is needed to keep the concentration of free ClO2 down.
There are some that seem to be very sensitive to ClO2. They seem to experience problems even at 1 - 2 drops. I don't know what the cause of this is, however most people tolerate a 1 PPM free ClO2 solution without problems.