I am new to Karate framework and have written the following working code to run few sample API tests.
Feature: Sample API Tests

  Background:
    * def request_headers = {apikey: "#(apikey)", Content-Type: 'application/json'}
    * configure headers = request_headers

  Scenario: Create Customer

    * def requestPayload = read('classpath:payload/customerdetails.json')
    * def RandomDataGenerator = Java.type('utils.RandomDataGenerator')

    Given url testurl+'/customers'
    And request requestPayload
    * requestPayload.firstName = RandomDataGenerator.generateRandomFirstName()
    * requestPayload.lastName = RandomDataGenerator.generateRandomLastName()
    * requestPayload.mobileNumber = RandomDataGenerator.generateRandomMobileNumber()
    * requestPayload.emailAddress = RandomDataGenerator.generateRandomEmailAddress()
    When method post
    Then status 200

    And def customerId = response.id
    
    # Create An Account for the customer
    * def createAccountPayload = read('classpath:payload/accountdetails.json')

    Given url testurl+'/accounts/createCreditAccount'
    And request createAccountPayload
    * createAccountPayload.customerId = customerId
    When method post
    Then status 200

    And def accountId = response.id

  # Perform a Transaction
    * def debitTransactionPayload = read('classpath:payload/transaction.json')

    Given url testurl+'/accounts/createDebitTransaction'
    And request debitTransactionPayload
    * debitTransactionPayload.accountId = accountId
    When method post
    Then status 200
    

Now I want to run a test where I would:

Create 100 customers (and in turn 100 accounts).
Do 500 transactions (5 transactions each for a given customer).

How do I achieve this?
I tried using repeat and looping options, but I believe I am missing something basic. Can someone please help ?

Repeating and looping API calls in Karate framework
Here is a Karate test that uses a JS function as a data-source to create 10 JSON payloads. You should be easily able to modify this to do what you want:
Feature:

  @setup
  Scenario:
    * def generator = 
    """
    function(i){ 
        if (i == 10) return null; 
        return { name: `cat${i}`, age: i };
    }
    """

  Scenario Outline:
    * print __row

    Examples:
        | karate.setup().generator |

How this works is explained here: https://github.com/karatelabs/karate#json-function-data-source
Also refer: https://stackoverflow.com/a/75394445/143475
