Question

How number of batteries affects bulb brightness?

Answer 1

Well batteries are full of energy the more batteries more light from the bulb. But sometimes the way the batteries are in can cause change in a effect and reaction

Answer 2

There's not enough information in the question to answer it.

-- If the batteries are in series, then
more batteries ==> brighter bulbs.

-- If the batteries are in parallel, then the number of batteries
has no effect on the brightness of the bulbs.

These two predictions apply equally well whether the bulbs
are in series or in parallel.