The numbers on the labels of e-liquid with nicotine represents the concentration, or strength, of nicotine in the e-liquid.
It is measured in milligrams of nicotine per millilitre of e-liquid (mg / ml) and is often shortened to mg. The strength can also be expressed as a percentage of nicotine by mass, e.g. 12mg/ml can be expressed as 1.2% nicotine or 12mg nicotine, and they both mean the same.
A 30ml bottle with a nicotine concentration of 12mg/ml will therefore contain a total of 360mg of nicotine (30ml x 12mg/ml). If 1ml of e-liquid is equal to about 20 drops, then there is about 0.6 mg of nicotine in each drop (12mg/ml ÷ 20 drops). So, if you load 10 drops of e-liquid in your e-cigarette, you should be consuming about 6 mg of nicotine (0.6 mg/drop x 10 drops) if you vape away those 10 drops.