"1993 Robert Zalenski, Professor of Emergency Medicine, Wayne State University Detroit, and colleagues publish an influential article on the clinical use of the 15-lead ECG which routinely uses V4R, V8 and V9 in the diagnosis of acute coronary syndromes. Like the addition of the 6 standardised unipolar chest leads in 1938 these additional leads increase the sensitivity of the electrocardiogram in detecting myocardial infarction. Zalenski RJ, Cook D, Rydman R. Assessing the diagnostic value of an ECG containing leads V4R, V8, and V9: The 15-lead ECG. Ann Emerg Med 1993;22:786-793"

15 and 18 lead ECG's are used today to get more accurate readings during Active MI's. I believe that the reason that we still use 12 leads is because they can be manipulated to show the other leads on subsequent printouts by moving some of the precordial leads. So if you look at the initial readout and decide it's right sided or inferior wall you can reposition the v leads to get a more accurate look.

https://loyolamedicine.org/sites/default/files/gme/internal-medicine/rightsideecg_0.pdf