The Entry No (entry number) item in the Call Stack window is a column of numbers that indicate the order of functions. The function with the highest number is the currently executing function; the one with the next highest number is its caller. Depending on the growth direction of the Call Stack window, entry numbers appear in descending or ascending order. The smallest entry number is always 1.
Select Entry No from the Options->Display style dialog for the Call Stack window if you want a column showing entry numbers to be displayed.