Which finger types which number?
In standard touch typing each finger reaches straight up from its home key: the left little finger types 1, ring 2, middle 3, and the left index covers 4 and 5. The right index covers 6 and 7, middle 8, ring 9 and little finger 0. Many typists drift to the wrong finger here, and that is the main reason number speed lags behind letter speed.
If you enter long runs of figures, the numeric keypad is usually faster. Try the 10 key typing test for that.