How do I implement for loop in this telco billing program in C?

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C

I have got the calculations all done. This is for my assignment. It is mandatory to include a for loop in this program. I can also include a do...while loop but the do...while is an optional one. I want to make sure this program can calculate as many bills as it can. For example, I want to pay for other people so the program will ask, "How many bills do you want to pay?" and I will key in the amount (eg: 2) & the program will run twice before terminating. The lines that I commented are my failed attempts btw. That is the only problem I have now. Thank you.

What I have tried:

#include <stdio.h>
#include <stdlib.h>

void main()
{
    int a, count, num, planType, postpaid, prepaid, minCall, msgSent;
    float billRate, callRate = 0.06 , smsRate = 0.01, totalBill ;
    char custName[100], phoneNum[20];

    printf("Enter your name: ");
    gets(custName);
    printf("\nEnter your phone number: ");
    scanf(" %s", &phoneNum);
    printf("\nWhich phone plan do you want to choose? (1 - postpaid, 2 - prepaid) ");
    scanf(" %d", &planType);


    if (planType == 1) {

   /* for loop execution */
        //for( a = 1; a <= 1; a = a + 1 ){
        //printf("\nHow many bills do you want to pay? ");
        //scanf(" %d", &a);
        //}
        
    /*for loop trial*
        int num, count, sum = 0;

    printf("Enter how many bills you want to pay: ");
    scanf("%d", &num);

    // for loop terminates when n is less than count
    for(count = 1; count <= num; ++count)
    {
        sum += count;
    }*/

        printf("\nWhich postpaid plan do you want? (98, 128, 158, 188) ");
        scanf(" %d", &postpaid);
    }
        if (postpaid == 98) {
            printf("\nSubscribed to MaxisONE Plan 98!\nPlan includes 15GB All Day Data, 15GB Weekend Data and unlimited calls & SMS");
            billRate = 98.00;
        }
        else if (postpaid == 128) {
            printf("\nSubscribed to MaxisONE Plan 128!\nPlan includes 40GB All Day Data and unlimited calls & SMS");
            billRate = 128.00;
        }
        else if (postpaid == 158) {
            printf("\nSubscribed to MaxisONE Plan 158!\nPlan includes 50GB All Day Data and unlimited calls & SMS");
            billRate = 158.00;
        }
        else if (postpaid == 188) {
            printf("\nSubscribed to MaxisONE Plan 188!\nPlan includes 60 GB All Day Data and unlimited calls & SMS");
            billRate = 188.00;
        }
     else {
        printf("\nWhich prepaid plan do you want? (35, 45, 60, 70) ");
        scanf(" %d", &prepaid);
        switch(prepaid) {
        case 35 :
            printf("\nSubscribed to Hotlink RED Plan 35!\n30-day validity Plan which includes 6GB All Day Data, free Non-Stop RED Chat or free Non-Stop RED Social");
            billRate = 35.00;
            break;
        case 45 :
            printf("\nSubscribed to Hotlink RED Plan 45!\n30-day validity Plan which includes 8GB All Day Data, free Non-Stop RED Chat or free Non-Stop RED Social");
            billRate = 45.00;
            break;
        case 60 :
            printf("\nSubscribed to Hotlink RED Plan 60!\n30-day validity Plan which includes 6GB All Day Data, unlimited calls & SMS, free Non-Stop RED Chat or free Non-Stop RED Social");
            billRate = 60.00;
            break;
        case 70 :
            printf("\nSubscribed to Hotlink RED Plan 70!\n30-day validity Plan which includes 8GB All Day Data, unlimited calls & SMS, free Non-Stop RED Chat or free Non-Stop RED Social");
            billRate = 70.00;
        }
    }

    printf("\n ");
    printf("\n===================================================\n");
    printf("\t\tMAXIS eBILL");
    printf("\n===================================================\n");
    time_t t = time(NULL);
    struct tm *tm = localtime(&t);
    printf("Date & Time: %s\n", asctime(tm));
    printf("Name: ");
    puts(custName);
    printf("Phone Number: %s", phoneNum);
    printf("\nPlan Type: %d", planType);

    if (planType == 1) {
        printf("\nPostpaid Plan: %d", postpaid);
        printf("\nTotal Bill: RM %.2f", billRate);
        printf("\n====================================================\n");
    }
    else {
        printf("\nPrepaid Plan: %d", prepaid);
        printf("\nHow many messages sent this month? ");
        scanf(" %d", &msgSent);
        printf("How long are your calls this month? (minutes) ");
        scanf(" %d", &minCall);
        totalBill = (callRate*2*minCall) + (smsRate*msgSent) + billRate;
        printf("Total Bill: RM %.2f", totalBill);
        printf("\n===================================================\n");

    }
}