Can anyone help me troubleshoot my code for the CS50 Credit problem? (**not trying to cheat, just need some help!)

Question

This is the code I have written in c for the cs50 credit pset 1 problem.

I am checking the length of numbers entered, and the first digits (AMEX 15 digits, starts with 34 or 37; MC 16 digits, starts with 51/52/53/54/55; VISA, 13 or 16 digits, starts with 4) and then applying Luhn's algorithm to see if a card is valid (every other digit x 2, add the digits of those multiplications together, and then add to the total of the digits not x2. If = 20, card is valid).

The trouble I am having is with the Luhn's algorithm section; there keeps being errors, and every time I try to fix these, others appear.

#include <cs50.h>
#include <stdio.h>
#include <math.h>

int main(void)
{
    long num = get_long("Number: ");
    long number = num;
    int digit;
    int x;
    int total_sum;
    int num_digits = 0;
    do 
    {
        num_digits ++;
        num /= 10;
    }
    while (num!=0); 
//invalid if not the right number of digits    
    if ((num_digits != 13) && (num_digits != 15) && (num_digits != 16))
    {
        printf("INVALID\n");
    }
    else
    { //check algorithm
    //if algorithm is false print invalid, otherwise the other lines of code become useful
    
       while (x != 0)
       {
            x = number / 10 ;
            int  z = number % 10;
            int  y = x % 10; //number in second last position;
            number = number/10; 
            if (y < 9)
            {
                total_sum = total_sum + 2*y + z;
            }
            else 
            {
                total_sum = total_sum + (y % 10) + (y /10) + z;
            }
       }
        
        if (total_sum % 10 != 0)
            {
                printf("INVALID\n");
             }
    
    //here, need to check luhn's algorithm. if false print invalid. if true, continue onwards.
        else if (number / 10000000000000 == 34  || number / 100000000000 == 34)
            {
            printf("AMEX\n");
            }
        else if (number / 100000000000000 == 4  || number / 1000000000 == 4)
            {
            printf("VISA\n");
            }
        else if (number / 100000000000000 == 51  || number / 1000000000000 == 52 || number / 1000000000000 == 53 || number / 1000000000000 == 54 || number / 1000000000000 == 55 )
            {
            printf("MASTERCARD\n");
            }
        else
        {
            printf("INVALID\n");
        }
        }
}