Setting Quantity Field on Subscriptions
I realize that this is an older question, but I have found the answer for it after much searching, and want to leave this answer for others who might be coming from a search engine.

The way I have found that you would do this is with the Stripe Payments plugin. The documentation for it can be found at https://github.com/craftcms/commerce-stripe. 

From there, you will find that there is a beforeSubscribe event. You can use that to set parameters for quantity. From the documentation:

use craft\commerce\stripe\events\SubscriptionRequestEvent;
use craft\commerce\stripe\base\SubscriptionGateway as StripeGateway;
use yii\base\Event;

Event::on(StripeGateway::class, StripeGateway::EVENT_BEFORE_SUBSCRIBE, function(SubscriptionRequestEvent $e) {
    $e-&gt;parameters['someKey'] = 'some value';
    unset($e-&gt;parameters['unneededKey']);
});


For my subscription, I have found that $e-&gt;parameters['items'][0]['quantity'] = 3; works for my setup.