gistfile1.php

/**
 * Restrict quantity of download at checkout
 */
function sumobi_edd_restrict_download_quantity( $valid_data, $post ) {

	$download_id = 50;
	$max_number  = 4;

	$cart_contents = edd_get_cart_contents();

	if ( $cart_contents ) {
		foreach ( $cart_contents as $download ) {
			if ( $download['id'] == $download_id ) {
				if ( $download['quantity'] > $max_number ) {
					edd_set_error( 'invalid_quantity', sprintf( __( 'You cannot purchase more than %s of this download', 'edd' ), $max_number ) );
				}
			}
		}
	}

}
add_action( 'edd_checkout_error_checks', 'sumobi_edd_restrict_download_quantity', 10, 2 );