well its depends on how much power you want.... how much bass you want too.... 2 is better then 1 in most cases unless there different models.... usualy 2 12's is pretty good for most poeple who just use there car for casuall driving like to work and where ever else.... poeple who go into comps like 15's....
if you have the money then i would suggest 15's.... my boi got a sick deal on an alpine r type 15inch sub lol, 2000W max and 750 Rms.... and he got a 800W mono amp with it. the guy paid over 700 for it all.... my boi traded him a 10in pioneer sub... and a 150W kicker amp hahah... the guy is now selling that for 80$... if you have a CC then check this site out
http://www.sonicelectronix.com/ amzaing prices on almost evertying... like that 15 inch alpine is only 189.99... they got dual kicker L7's for like 600 in a box.... if you really are rich they got 18inch kickers for 699 each... 10000 max power and 5000 rms... im sure those would pound lol