The number of electrodes does not determine the number of leads. In fact, it takes at least 2 electrodes to make 1 lead. The most common and useful lead, lead 2, looks at the hearts electrical activity from the negative right arm electrode to the positive left arm electrode. Because the electrical activity is moving from negative to positive, the ECG reading is upright which is in contrast to other leads. With 12-leads, the unipolar limb leads are used with precordial (chest) leads and augmented limb leads to give 12 total leads. It is best that you learn three leads first and then try and learn about 12 leads. That said, I will give you a little taste of 12-leads by explaining where every lead comes from. Note, the right leg electrode acts as a ground and is not a lead!
1) Lead 1 = Right arm electrode to Left arm electrode
2) Lead 2 = R arm electrode to L leg electrode
3) Lead 3 = L arm electrode to L led electrode
4) Augmented Limb lead Right arm aVR = R arm to spliced L Arm and L Leg
5) aVL = L arm to spliced R arm and R Leg
6) aVF (F=foot) = L let to spliced R arm and L arm
7) Precordial leads use aVR with V1
8) aVR with V2
9) aVR with V3
10) ditto V4
11) ditto V5
12) ditto V6
There you have it, 12 leads. I think I got them all right but then again its late and I'm tired. I'm sure I'll hear about it if I got one wrong however. BTW, if you don't completely understand 12-leads now, don't feel bad. This was just meant to wet your appetite and really is only the beginning.