The up-down arrow keys seem like reasonable keys to use for incrementing or decrementing a number box or a slider. But rather than require a new key press for each step up or down, you can use a fast metro to trigger messages continuously as long as the key is pressed. There are several ways one could implement this in Max; here's one example.
Using both the key and keyup objects, you can tell when a key was pressed and when it was released. The split object allows you to isolate a range of numbers; it passes only the specified range of numbers out its left outlet, and passes all other numbers out its right outlet. The numbered keys 0 to 9 correspond to ASCII codes 48 to 57, so it's easy to isolate those keys, and subtracting 48 brings those numbers down into the range 0 to 9.