To add some extra technical information, there is almost no actual increase in volume when you add more speakers without increasing the amplifier output power. In other words, although you have more cone surface to move the air, the power is now being divided between a larger number of speakers. However, two other effects come into play, which make the overall sound appear to be louder.
The first is that as you add more speakers, the sound from the cab becomes more directional and focused into a beam. So that provided you're standing towards the front of the cab, what you hear will be louder. The more speakers you add, the tighter and more directional the beam becomes. A 1X12 tends to spread sound all around, while a 2X12 spreads it around on one axis and focuses it along the other. A 4X12 focuses the beam along both axes. If you want a 2X12 to spread sound across a stage or auditorium, then you need to stand it with the speakers above and below each other. Then it will also spread less sound above and below the cab.
The second is a lesser effect, and happens if the amplifier is pushing the the speaker(s) towards the limit, so they start to compress and breakup. When you add more speakers, so that each speaker is handling less power, then it will operate in its more linear region, so the cone moves more freely and can put more sound out.
You can add this to the party bores' encyclopedia of useless information....