If bulb connected as series, all of the battery voltage spread with each bulb. So if you exceed the voltage limit it will cause dim the each light.but it will consume the same current amount among all. If you connected same value bulbs it will give same lights, if it tend to dim, you need to increase the voltage amount according to bulbs.
If bulbs connected as parallel, it will consume same voltage, so all of the bulb give same light.but it will consume more current amount when exceed some bulb amount.If you connected same value bulbs it will give same lights, if it tend to dim, you need to increase the current amount.