<?php
// only queue when page slug where CF is located is true

function contactform() {
    if(is_page(‘page-slug’)) {
        wp_register_script( 'jerrixit_contactform', get_stylesheet_directory_uri() . '/jerrixit.js', array( 'jquery' ) );
        wp_enqueue_script( 'jerrixit_contactform' );
    }
}
add_action( 'wp_enqueue_scripts', 'contactform' );

// .JS file

jQuery(function($){
    var max = 20;
    var checkboxes = $('input[type="checkbox"]');
    checkboxes.change(function(){
        var current = checkboxes.filter(':checked').length;
        checkboxes.filter(':not(:checked)').prop('disabled', current >= max);
    });
});