Change quantity input field to a Dropdown in WooCommerce

Caution: First you should never overwrite WooCommerce core files, for many reasons. So it’s prohibited.

Instead as woocommerce_quantity_input() function call the template file global/quantity-input.php, you can override that template via your child theme.

To understand how to override templates, read carefully: Overriding templates via a theme in WooCommerce.

Now, remove all your related quantity changes and code from you web site (restore everything as before).

Then copy quantity-input.php file located inside WooCommerce plugin > templates > global, to your child theme into a "woocommerce" folder > "global" subfolder.

Once done, open / edit it, and replace the template content with:

<?php
/**
 * Product quantity inputs
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/global/quantity-input.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see     https://docs.woocommerce.com/document/template-structure/
 * @package WooCommerceTemplates
 * @version 4.0.0
 */

defined( 'ABSPATH' ) || exit;

if ( $max_value && $min_value === $max_value ) {
    ?>
    <div class="quantity hidden">
        <input type="hidden" id="<?php echo esc_attr( $input_id ); ?>" class="qty" name="<?php echo esc_attr( $input_name ); ?>" value="<?php echo esc_attr( $min_value ); ?>" />
    </div>
    <?php
} else {
    /* translators: %s: Quantity. */
    $label = ! empty( $args['product_name'] ) ? sprintf( esc_html__( '%s quantity', 'woocommerce' ), wp_strip_all_tags( $args['product_name'] ) ) : esc_html__( 'Quantity', 'woocommerce' );
    ?>
    <div class="quantity">
        <?php do_action( 'woocommerce_before_quantity_input_field' ); ?>
        <label class="screen-reader-text" for="<?php echo esc_attr( $input_id ); ?>"><?php echo esc_attr( $label ); ?></label>
        <?php
        if ( is_cart() ) :
        ?>
        <input
            type="hidden"
            id="<?php echo esc_attr( $input_id ); ?>"
            class="<?php echo esc_attr( join( ' ', (array) $classes ) ); ?>"
            name="<?php echo esc_attr( $input_name ); ?>"
            value="<?php echo esc_attr( $input_value ); ?>"
            title="<?php echo esc_attr_x( 'Qty', 'Product quantity input tooltip', 'woocommerce' );
        ?>" />
        <?php
        endif;

        $options = ''; // Initializing

        for ( $i = $min_value; $i <= $max_value; $i += $step ) :
            $selected = ( '' !== $input_value && $input_value >= 1 && $i == $input_value ) ? 'selected' : '';
            $options .= '<option value="' . $i . '"' . $selected . '>' . $i . '</option>';
        endfor;
        // Change input name on select field
        $attr_name = is_cart() ? 'data-name' : 'name';
        ?>
            <select <?php echo $attr_name; ?>="<?php echo $input_name; ?>"><?php echo $options; ?></select>
        <?php do_action( 'woocommerce_after_quantity_input_field' ); ?>
    </div>
    <?php
}

Now some jQuery code is required, to make things work on cart page.

// jQuery - cart jQuery script for quantity dropdown
add_action( 'woocommerce_after_cart', 'cart_quantity_dropdown_js' );
function cart_quantity_dropdown_js() {
    ?>
    <script type="text/javascript">
    jQuery( function($){
        $(document.body).on('change blur', 'form.woocommerce-cart-form .quantity select', function(e){
            var t = $(this), q = t.val(), p = t.parent();
            $(this).parent().find('input').val($(this).val());
            console.log($(this).parent().find('input').val());
        });
    });
    </script>
    <?php
}

This code goes in functions.php file of the active child theme (or active theme).

Now to restrict the max quantity to 6, add the following code:

// Restricting product max quantity to 6  
add_filter( 'woocommerce_quantity_input_args', 'woocommerce_quantity_input_args_callback', 10, 2 );
function woocommerce_quantity_input_args_callback( $args, $product ) {
    $args['max_value'] = 6;

    return $args;
}

// Restricting product variation max quantity to 6
add_filter( 'woocommerce_available_variation', 'filter_woocommerce_available_variation', 10, 3);
function filter_woocommerce_available_variation( $data, $product, $variation ) {
    $data['max_qty'] = 6;

    return $data;
}

This code goes in functions.php file of the active child theme (or active theme).

Now it works everywhere (tested on last WooCommerce version under Storefront theme).