Nicotine in e-liquid is typically measured in mg/ml, with bottles commonly coming in levels of 0mg, 3mg, 6mg, 12mg and 18mg. This simply tells you the amount of the liquid nicotine there is in a bottle, in relation to the total amount of e-liquid in the bottle.
This means that a bigger bottle of e-liquid will contain more nicotine overall than a smaller bottle in the same nicotine strength. So if you get a 15 ml bottle and a 30 ml bottle in the same strength, the 30 ml bottle will contain twice as much nicotine as the 15 ml bottle.
The nicotine is mixed throughout the bottle, though, so whenever you fill up the same tank or clearomizer with e-liquid of a specific strength, you get the same amount of nicotine. In other words, in a bottle of 12mg/ml e-liquid, every ml of e-liquid contains 12 milligrams of nicotine, mixed evenly throughout so you get a consistent hit of nicotine with every puff.