A quick little code snippet that extends the in-built add_to_cart shortcode and adds a Quantity Box with it. Feel free to modify this code all you like.
Tested on WordPress 3.5.x and WP-E-Commerce 3.8.x

The original functions can be found in :
/wp-content/plugins/wp-e-commerce/wpsc-includes/display.functions.php (approx line 198)
/wp-content/plugins/wp-e-commerce/wpsc-includes/theme.functions.php (approx line 1196)

Just copy and paste the code into your functions.php file in your theme.
Then you can use the shortcode [cdl_add_to_cart=99] in your post or page, remember to replace the 99 with your own product ID.

/*-------------------------------------------------------------------------------------------*/
/* cdl_add cart button function used for php template tags and shortcodes
/*-------------------------------------------------------------------------------------------*/


function cdl_add_to_cart_button( $product_id, $return = false ) {
	global $wpdb,$wpsc_variations;
	$output = '';
	if ( $product_id > 0 ) {
		// grab the variation form fields here
		$wpsc_variations = new wpsc_variations( $product_id );
		if ( $return )
			ob_start();
		?>
			<div class='wpsc-add-to-cart-button'>
				<form class='wpsc-add-to-cart-button-form' id='product_<?php echo esc_attr( $product_id ) ?>' action='' method='post'>
					<?php do_action( 'wpsc_add_to_cart_button_form_begin' ); ?>
					<div class='wpsc_variation_forms'>
						<?php while ( wpsc_have_variation_groups() ) : wpsc_the_variation_group(); ?>
							<p>
								<select class='wpsc_select_variation' name='variation[<?php echo wpsc_vargrp_id(); ?>]' id='<?php echo wpsc_vargrp_form_id(); ?>'>
									<?php while ( wpsc_have_variations() ): wpsc_the_variation(); ?>
										<option value='<?php echo wpsc_the_variation_id(); ?>' <?php echo wpsc_the_variation_out_of_stock(); ?>><?php echo esc_html( wpsc_the_variation_name() ); ?></option>
									<?php endwhile; ?>
								</select>
							</p>
						<?php endwhile; ?>
					</div>
					
					<legend>Quantity</legend>
						<div class="wpsc_quantity_update">
							<input id="wpsc_quantity_update_<?php echo esc_attr( $product_id ) ?>" name="wpsc_quantity_update" size="2" value="1" type="text">
							<input name="key" value="-1" type="hidden">
							<input name="wpsc_update_quantity" value="true" type="hidden">
                        </div><!--close wpsc_quantity_update-->
                    
					
					
					<input type='hidden' name='wpsc_ajax_action' value='add_to_cart' />
					<input type='hidden' name='product_id' value='<?php echo $product_id; ?>' />
					<input type='submit' id='product_<?php echo $product_id; ?>_submit_button' class='wpsc_buy_button' name='Buy' value='<?php echo __( 'Add To Cart', 'wpsc' ); ?>'  />
					<?php do_action( 'wpsc_add_to_cart_button_form_end' ); ?>
				</form>
			</div>
		<?php
		
		if ( $return )
			return ob_get_clean();
	}
}

function cdl_add_to_cart_shortcode( $content = '' ) {
	static $fancy_notification_output = false;
	if ( preg_match_all( "/\[cdl_add_to_cart=([\d]+)\]/", $content, $matches ) ) {
		foreach ( $matches[1] as $key => $product_id ) {
			$original_string = $matches[0][$key];
			$output = cdl_add_to_cart_button( $product_id, true );
			$content = str_replace( $original_string, $output, $content );
		}

		if ( ! $fancy_notification_output ) {
			$content .= wpsc_fancy_notifications( true );
			$fancy_notification_output = true;
		}
	}
	return $content;
}
add_filter( 'the_content', 'cdl_add_to_cart_shortcode', 12 );