Here are five possible code completions for the given function:

### 1. Using `snprintf` to calculate the number of characters used by the number

```c
static void
init_parameters (int number_of_files)
{
  int chars_used_by_number = 0;
  char buffer[32];
  snprintf(buffer, sizeof(buffer), "%d", number_of_files);
  chars_used_by_number = strlen(buffer);
  // Rest of the function implementation...
}
```

### 2. Using `log10` to calculate the number of characters used by the number

```c
static void
init_parameters (int number_of_files)
{
  int chars_used_by_number = 0;
  if (number_of_files == 0) {
    chars_used_by_number = 1;
  } else {
    chars_used_by_number = (int) log10(abs(number_of_files)) + 1;
  }
  // Rest of the function implementation...
}
```

### 3. Using a loop to calculate the number of characters used by the number

```c
static void
init_parameters (int number_of_files)
{
  int chars_used_by_number = 0;
  int temp = abs(number_of_files);
  do {
    chars_used_by_number++;
    temp /= 10;
  } while (temp != 0);
  // Rest of the function implementation...
}
```

### 4. Using `itoa` to calculate the number of characters used by the number

```c
static void
init_parameters (int number_of_files)
{
  int chars_used_by_number = 0;
  char buffer[32];
  itoa(number_of_files, buffer, 10);
  chars_used_by_number = strlen(buffer);
  // Rest of the function implementation...
}
```

### 5. Using `sprintf` to calculate the number of characters used by the number

```c
static void
init_parameters (int number_of_files)
{
  int chars_used_by_number = 0;
  char buffer[32];
  sprintf(buffer, "%d", number_of_files);
  chars_used_by_number = strlen(buffer);
  // Rest of the function implementation...
}
```

Note: The `itoa` function is not a standard C function and may not be available on all platforms. The `snprintf` function is generally safer than `sprintf` as it prevents buffer overflows.