Please use this to get the number of ratings in the product page
<?php
$ratingModel = VmModel::getModel('Ratings');
$productrating = $ratingModel->getRatingByProduct($this->product->virtuemart_product_id);
?>
<span itemprop="ratingCount"><?php echo $productrating->ratingcount; ?></span>
Btw structured data is already available in the product page if you use the default virtuemart theme but you can also easily added to your template by adding this code in the product view
<?php
if ($this->product->prices['salesPrice'] > 0) {
echo shopFunctionsF::renderVmSubLayout('snippets',array('product'=>$this->product, 'currency'=>$this->currency, 'showRating'=>$this->showRating));
}
?>