Box and folder numbers are followed by a number in parenthesis which indicates the number of items by (or to) that person. So in this example

Eberhart, Richard, 1904- --19.4 (3), (3 from Sexton), 30.2 (1 to Anne Ford, 1 to Carolyn Amussen)

there are three letters to Anne Sexton and three letters from Sexton in box 19, folder 4 and there is one letter from Eberhart to Anne Ford and one letter to Carolyn Amussen in box 30, folder 2.