THE ANSWER:
As described above, 20 points is the most common two-card blackjack score; 146 different two-card combinations result in a score of 20.
Second on the list is 13; there are 118 different two-card combinations that total 13.
Third on the list is 14; some 102 different two-card combinations give a score of 14.
(For the record, a score of 21 can occur from 64 different two-card combinations.)