...
The function how-many-acc returns immediately to the caller after making its recursive call (how-many-acc (rest lox) (+ acc 1)), without having to perform any additional computation. The function how-many-acc updates its argument acc (by adding 1 to it) efore passing it on to the recursive call. By the time the recursive call reaches the end of the list, marked by empty, it simply returns the argument acc. The function how-many-acc uses its acc parameter to "accumulate" the result of its computation. As such the parameter acc is called an accumulator.
We now can make use of how-many-acc to write a highly efficient function to compute the number of elements in a list.
...