Final answer:
The number of electrons flowing from cathode to anode increases linearly as the milliampere doubles, adhering to Ohm's law which states that current is directly proportional to the voltage if resistance is constant.
Step-by-step explanation:
As the milliampere (mA) doubles, the number of electrons flowing from cathode to anode increases linearly. This is because electric current is directly proportional to the charge flow per unit of time, and if the current is doubled, the number of electrons, which are the carriers of charge, also doubles. This concept is in accordance with Ohm's law, where the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them. If resistance remains constant and the current doubles due to an increase in voltage or a decrease in resistance, the flow of electrons which makes up the current also doubles.