Does adding more speakers change the sensitivity of a cabinet? Is there an equation for this? Say i have a 100db rating at 1K Hz for 1 particular speaker...what would happen with 2 speakers or 4?

It doesnt change the sensetivity for the cab but you would produce more volume because of more speakers etc. I think.

If you double the number of drivers (or speakers, which is the finished product, not what's inside) parallel wired with an equal voltage input you will gain 6dB of sensitivity. 3dB comes from the increased radiation efficiency resulting from doubling the radiating area, 3dB comes from the doubling of the wattage draw resulting from halving the impedance load. Double it again and you'll get another 6dB, if you haven't exceeded the amperage draw capability of the amp by going too low with the impedance.

It assumes that they are identical, and that the enclosure is also doubled in size to maintain the same tuning.

So, if I had a 10" speaker that had a rating of 96dB at 200HZ (1 watt/one meter) then making a cab a 2x10" would get me a 102dB rating at the same frequency? Therefore, a 4x10" with the same drivers would be 108dB? Am I doing that right?

Yes and no. The 6dB gain comes in two lots. The first lot of 3dB gain is in the speaker cab only. A 2x10 with two speakers of 96dB sensitivity, the overall cab sensitivity increases by 3dB. So it's a 99dB cab. The other 3dB comes from the change in nominal impedance. Lets assume they're 8 ohm speakers and the amp has a 4 ohm min. With one speaker, the amp sees an 8 ohm load and therefore produces less than full power. Adding the second speaker in parallel means the amp sees a 4 ohm nominal impedance instead, so tha amp can produce full power. That's your second lot of 3dB.