driving subs/enough power?

It is impossible to answer that question since it depends much more on the enclosure and the installation configuration than on simply increasing cone area.  In general, though, if you move from 2 speakers to 4 speakers with the same power, you can add +3db to your SPL total.  If you double the power to 2 speakers, you can also add +3db.  If you double both the speaker number and the power level, you add +6db.  (in case you do not know, +10db = twice as loud.)  But like I said, that is theoretical and it depends on the way the system is built.

so what about if you have 2-12's in a ported box with 500 watts total, if you put in 2-15's with the same amp and similar box, will you increase the amount of sound? I also, am trying to decide on going from 2-12's up to either 3-12's or else 2-15's,

I understand if I double the power of the amp I will get more sound also.

Again, it is impossible to say with any certainty what will happen.  In general any time you increase the available cone surface area you in theory increase the SPL capability, but there are so many other factors to consider that any predictions based solely on the sizes of the woofers is likely to be incorrect.  Changing from dual 12" woofers in a properly sized and aligned enclosure to dual 15" woofers of the same type (brand, sensitivity, etc.) in a properly sized and aligned enclosure but using the same amplifier will likely increase the SPL, yes.  How much and in what frequency bands is impossible to say.

No it is not.  Who told you that?