If you take the simplest form of capacitor, two parallel plates, the the capacitance is proportional to the area of the plates and inversely proportional to the distance between the plates:
$$C \propto \frac{A}{d}$$
When you're making a capacitor out of a snapple bottle you're actually making something similar to the simple "two plate" capacitor but the "plates" are curved round the surface of the bottle, with the foil as the external plate and the (conducting) salt water in the bottle acting as the internal plate. The $d$ in the equation above is the thickness of the glass.
So you can make a capacitor out of any bottle, jar or anything similar. All that matters is the area of the foil and the thickness of the glass. If you want to increase the total capacitance you can just link any number of bottles in parallel i.e. link the external foil covers as one electrode and the internal brine solution as the other electrode. When you join up capacitors in this way, "in parallel", you get the total capacitance just by adding up the individual capacitances of all the bottles you've joined.
You ask about the effect of the increased voltage: the charge stored in a capacitor is given by:
$$Q = CV$$
where $C$ is the capacitance and $V$ the voltage, so using 12kV instead of 9kV just means you get 33% more charge for a given capacitance, or alternatively you can get away with a smaller capacitance to hold the same charge.
Do I sound a bit like a grandma if I point out you need to be careful with this experiment. A typical Tesla coil can't generate enough current to kill you, but if you gang together enough capacitors the stored charge in them will kill you!
Finally, I normally point people to Wikipedia if they want to learn more, so see http://en.wikipedia.org/wiki/Capacitor and http://en.wikipedia.org/wiki/Leyden_jar for the sort of capacitor you're making. However be warned that the Wikipedia article on the capacitor is a bit technical.
