The outer dead layers of the skin are pretty good insulators when intact and dry (10 or 20 kohm or more). But get your skin wet or punctured and you are basically a sack of salt water. A rather good conductor, a couple of hundred ohms maybe. Now at 12 v you aren't likely to get hurt, as the current would be limited to perhaps 1/20 amp, and would have a sec or more to get loose. As a rule of thumb I start to get nervous around 30 or 40 v.
Now the path through your body is important. The danger is mainly current flow through the heart muscle. In one hand and out the other or in one hand and out a foot, is realy bad. Which is why we are also taught to wear rubber soled shoes, stand on dry floors, and to always keep one hand in our pocket when probing about in high v /high energy circuits.
And _never_ intensionally take a shock. It's kind of a matter of professional pride among etechs to be able to say 'oh, its been years sinced I last got shocked'. In fact, some employers would fire your butt for intensionally taking a shock.