More speakers = More Noise ?’s

Does adding more speakers actually increase volume?   What would be the actual difference between having 8 speakers running at 50 Watts rms each or having only 4 running at 50 Watts rms each?  More SPL right?  Does more SPL necessarily mean more volume though?  SPL is sound pressure level, so is this actually a measurement of volume necessarily?  If i have a hundred speakers in my car but the volume on my head unit at 5, will it still sound quite?  I think it will, but will it have a higher SPL at this volume.  Also what are the actual affects of more subwoofers in comparison to those of the same amount of subwoofers just with more power.  I've read that to double the volume of your subs you must give them ten times the power.  So what does doubling the cone space by adding a second sub do?  It will make it hit twice as hard right, but is that just the feeling that hits twice as hard at the same volume?  In this case, spl would be hard to gain by adding High and Midrange speakers, right, since they don't produce that pounding feeling, just volume. 

Sorry for so many questions, thanks.

SPL = loudness.  So more SPL will by definition sound "louder."  In general, adding a second equal driver at the same power level to a system adds about +3db to the SPL.  This is not a linear additive effect, however, so adding 100 speakers does not mean +300db.  I m not good enough in calculus to answer your question about 100 speakers accurately, but in general will it still "sound quiet"?  No, it would sound about 10 times as loud.  And yes, what you heard is basically correct, doubling the perceived loudness of a sound (i.e. increasing the SPL from 50db to 100db) would require about 10X the amount of acoustical power, whether that was achieved by adding more cone area (more speakers) or by adding more amplifier power to the same speaker does not matter, you could do it either way.  So when the stereo salesman tells you that 250 watt amp will sound twoce as loud as that 125 watt amp, tell them to go back to school!

Once you hit the max rating on a sub, then yes, adding more speakers is the only way to create more SPL.  The SPL a sub can create depends on the speaker ratings, the enclosure design, speaker placement, AND available power.  Proper enclosure design can do more to add to your SPL (or improve your SQ) than almost anything else in the system.  After that, the next thing to do if all you are after is SPL, in my opinon, is to max out the power.  And remember, "doubling the power" or doubling the speakers has nothing to do with doubling the SPL. 

The actual formula for calculating net SPL from multiple speakers is SPLnet=10 x log ((10^SPL1/10) + (10^SPL2/10) + (10^SPL3/10)) etc etc....  So, if your first sub can produce, say, 100db SPL at max power, adding a second sub exactly like it (that also has max power available) will raise total SPL to about 103db.  Will you hear that difference?  Maybe, maybe not.