Answer:
Brightness will decrease
Step-by-step explanation:
If Bulbs are connected in series then here we can say bulbs behaves like resistance in the circuit so if we add more bulbs in series then it will add more resistance in the circuit.
Due to this resistance addition in series we will have less current in series circuit.
As we know by ohm's law
V = i R
if more bulbs are connected in series then resistance will be more and due to which current in the circuit will decrease
Now the intensity of light will depends on power consumed by the bulb which is given as

so here since current is decreased so power in the circuit will also decrease so intensity of light will go down