Very simply: if it's an even number, it's your left hand. If it's an odd number, it's your right hand. If you want to type a number like 2,467,333 you can do it without looking for the numbers. On qwerty I tend to "lose my footing" when it comes to typing numbers out. But if it's a 2 on programmer-dvorak, it's right hand, index finger. 2468 is each of your four digits, in order. and 1357 is the other hand's digits. However, I hold left-shift to access the numbers which means my pinky is unavailable for the left numerical ascension, so I tend to do 1 and 3 with my index, and 5 with middle, and 7 with ring. Nine [9] and Zero [0] are "stretch" numbers in the middle: 9 and 0 are reached via one's index fingers going diagonally toward the middle of the tippy-top row.

tl;dr: right hand even,, left hand odd,,, increasing from index to pinky. "Look ma, no eyes!"