When using WooCommerce you may want to limit the number of items that can be initially placed in the cart, or can be increased to while already in the cart.

Place the following code anywhere in your child theme’s functions.php document.

// limiting the number of items in cart when adding to the cart
function limit_number_of_items_allowed_cart_added( $passed, $product_id, $quantity ) {

    $cart_items_count = WC()->cart->get_cart_contents_count();
    $total_count = $cart_items_count + $quantity;

    if( $cart_items_count >= 99 || $total_count >99 ){
        // set to false
        $passed = false;
        // display a message
         wc_add_notice( __( 'If you require a quantity of more than 99 products to be shipped, please <a href="/contact-us/">Contact Sales</a> in lieu of placing your order online. Thank you, and we apologize for any inconvenience.', 'woocommerce' ), 'error' );
    }
    return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'limit_number_of_items_allowed_cart_added', 10, 3 );

Place the following code anywhere in your child theme’s functions.php document.

// limiting the number of items in cart when updating within the cart
function limit_number_of_items_allowed_cart_update( $passed, $cart_item_key, $values, $updated_quantity ) {

    $cart_items_count = WC()->cart->get_cart_contents_count();
    $original_quantity = $values['quantity'];
    $total_count = $cart_items_count - $original_quantity + $updated_quantity;

    if( $cart_items_count > 99 || $total_count > 99 ){
        // set to false
        $passed = false;
        // display a message
         wc_add_notice( __( 'If you require a quantity of more than 99 products to be shipped, please <a href="/contact-us/">Contact Sales</a> in lieu of placing your order online. Thank you, and we apologize for any inconvenience.', 'woocommerce' ), 'error' );
    }
    return $passed;
}
add_filter( 'woocommerce_update_cart_validation', 'limit_number_of_items_allowed_cart_update', 10, 4 );