How to limit or set a max number of allowed checkbox selections? – Shopify website

Link to website: https://island-paradise-tours.myshopify.com/products/full-day-excursion-6-hours (password = paradiseTours)

I am working for the first time in Shopify and in hopes to find out how to limit the total number of checkboxes a user can select when they are going through the steps of selecting “What activities would you like to add?”

You will see this on the page after you select a date/time for the service. I need it to cap at 4 checkboxes max to just throw an alert. I can spice the design up later, but just need to catch an error.

Like I said, I’m using Shopify and I am using a plugin or app called Propel Appointment Booking. It allows you to add custom questions (i.e. Checkbox), but I can’t set a max limit.

Overall, the goal is to make sure the user doesn’t select more than 4 of the 11 checkboxes. And at the moment, I have no way of adding a specific ID or Class to the input field to call it. Little lost.

Code I am using:

window.onload = function () {
    const input = document.querySelector('input[type=checkbox]');
    function check() {
        if (input.checked) {
            alert("checked");
        } else {
            alert("You didn't check it.");
        }
    }
    input.onchange = check;
    check();
}