I want to replace part of the integer number first number with 1 and next 4/5 numbers with 0

I want to replace part of the ntiger number first number with 1 and next 4/5 numbers with 0. No changed in first number is 0.

For example,

1. I'm getting 8123456789 from backend, but I want to display it as 1000056789.
2. I'm getting 9665789456789 but I want to display it as 1000009456789.

Would like to take decision number of changing numbers(chracter) with a logic like if user start with A then would like to change first 6 numbers or if user start with B then would like to change first 5 numbers.

I don't fully understand the logic you are looking for but see this an example of the approach you can use to make the thing you actually want.

I recommend first splitting your number into a List<String> where each digit are represented as a character. By doing so, it makes it a lot simpler to modify individual digits and we can later on join them back to a number.

void main() {
  print(fixNumber(8123456789, amountOfZeroes: 4));    // 1000056789
  print(fixNumber(9665789456789, amountOfZeroes: 5)); // 1000009456789
}

int fixNumber(int number, {required int amountOfZeroes}) {
  List<String> digits = number.toString().split('');
  digits[0] = '1';
  
  for (var i = 1; i <= amountOfZeroes; i++) {
    digits[i] = '0';
  }
  
  return int.parse(digits.join());
}