You don’t want to annoy the admissions committee by sending excessive correspondence outside of their required application materials. We recommend sending one update letter when applicable (remember, in the cases where you are in limbo, and the medical schools allow update letters).
There are some medical schools that allow up to three update letters. Some medical schools allow letters of interest and letters of intent in addition to update letters. Please check with the medical school to which you have applied to ensure that you follow their application instructions.