ff_checkbox_max_restriction.js

// Add the JS code to your Forms custom JS section. 
// Add a container class with the name - max_1_items on your checkbox field.

function maxItemsCheck(containerClass, maxChecked) {
    const checkboxes = document.querySelectorAll('.' + containerClass + ' input');
    
    checkboxes.forEach(function(checkbox) {
        checkbox.addEventListener('change', function() {
            const checkedCheckboxes = document.querySelectorAll('.' + containerClass + ' input:checked');
            const count = checkedCheckboxes.length;

            if (count > maxChecked) {
                alert('You can not check more than ' + maxChecked + ' items');
                this.checked = false;
            }
        });
    });
}

// Example Use case
maxItemsCheck('max_1_items', 1);