If the number of password digits is uncertain, please add up each case.
If the number cannot be duplicated:
One-digit password: 9 kinds;
Two-digit password: 9*8 kinds;
Three-digit password: 9*8*7 kinds;
Four-digit password: 9*8*7*6 kinds;
And so on;
If you can't determine the number of digits, add up each situation.
This is the principle of mathematical permutation and combination for your reference.