Extract Active Subscription Quantity

Hello,

How can I get the quantity of a user's active subscription?

The closest thing I have found is:

Code:

$invoices = Am_Di::getInstance()->invoiceTable->findByUserId($user_id);
foreach ($invoices as $invoice) {
    var_dump($invoice->first_total);
}

And then, I can calculate the quantity based on the total price.

Surely there must be a better way though.