A small tweak for the WooCommerce plugin for WordPress - This sets the minimum number of items to be present in the cart for checkout. Here's the error message in action: http://imgur.com/74BEhW5

minimum_cart_count.php

/*
 * Include this code in your theme's functions.php
 */
 
add_action( 'woocommerce_checkout_process', 'wc_minimum_cart_count' );
 
function wc_minimum_cart_count() {
	global $woocommerce;
	$minimum = 6;
	if ( $woocommerce->cart->get_cart_contents_count() < $minimum ) {
		$woocommerce->add_error( sprintf( 'You must have a minimum of %s items in your cart to place an order' , $minimum ) );
	}
}