Trying to get the length of digits in c

Question

I'm trying to understand why I am getting this error message.

error: data argument not used by format string [-Werror,-Wformat-extra-args]
    printf("Enter card number: ", card_number);

here is the code that I'm trying to run the only difference between this and the code I'm copying is the name of the int.

#include <stdio.h>

int main ()
{
//Get card number from customer
    int card_number;
    int length = 0;

    printf("Enter card number: ", card_number);
    scanf("%d", &card_number);

    while(card_number != 0)
    {
        card_number /= 10;
        ++length;
    }

    printf("Digits: %d", length);

    return 0;
}

I'm trying to use this to check if the length of the card_number entered is equal to a certain value. I haven't coded anything else due to this error. If I can understand better why I'm getting this error it would be greatly appreciated.