Unfortunately, Subscriptions does not allow to specify quantity. The reason why it’s configurable via Products interface is because of a bug which is going to be fixed in the upcoming version.
It’s not our limitation, it’s the way Stripe handles Subscriptions. You cannot specify quantity for a plan.
Good news is that variable amount support is coming for Subscriptions addon where your customers would be able to enter subscription amount and (even subscription term if that is needed). Cannot give you ETA at the moment, but is planned to be implemented within a week or so.