Let's determine what happens when you add batteries in series.
There are two major ways to wire batteries together which are:
Series and arallel.
A circuit can be said to be in series when there is the same curent flowing through all the components in the circuit.
In a series circuit, when you add a battery, the voltage increases while the supply voltage is divided among the components in the circuit.
Therefore, when you add batteries in series, the voltage is divided among the bulbs equally.
ANSWER:
It is divided among the bulbs.