Stripe coding for product

We use Stripe as our merchant account and have a subscription based product which is through a Clickfunnels checkout page. I need the customer to have the ability to change the quantity from 1 unit to multiple subscription based units at the time of checkout. The customer should have the ability to add up to 20 "units" at the time of purchase.

Here's the Stripe instructions showing how to do this:

https://stripe.com/docs/billing/subscriptions/multiple-products#creating