10,000 numbers can be made from combinations of four digits from 0 to 9. The formula for counting the number of combinations is 10^4, or 10 x 10 x 10 x 10.
Only two numbers are required to figure out how many digits combinations are possible. First, calculate the number of places for digits, which, in this case, is four. Second, calculate the number of choices of digits, which, in this case, is 10. Since digits can be repeated in a combination, each of the four places for digits has 10 digit options: 10 choices for the first place, 10 for the second and so on. The total number of combinations is thus 10 x 10 x 10 x 10.