If we add two more bulb in a series what will happen to all of them?

When two additional bulbs are added in series to an existing circuit, the total resistance increases. This causes a reduction in the overall current flowing through the circuit, which results in all the bulbs receiving less voltage. Consequently, the brightness of all the bulbs will decrease as they share the reduced voltage. If the power supply cannot handle the increased load, it may also lead to dimming or flickering.