Final answer:
The quantity of photons produced per second is primarily controlled by the milliamperage (mA), as it directly influences the number of electrons that generate photons during X-ray production.
Step-by-step explanation:
The quantity, or number of photons produced per second during an exposure is primarily controlled by the milliamperage (mA). This is because milliamperage directly affects the number of electrons interacting with the target material in an X-ray tube, thereby altering the number of photons generated. Factors such as exposure time, kilovoltage (kVp), and source-to-image distance (SID) can influence other aspects of the radiation produced, such as its duration, energy, and intensity, but the milliamperage is the key determinant of the quantity of photons.