Heroku maximum monthly charge with Hobby plan?

I have always been a free Heroku user however I recently changed an app to run on the Hobby plan. I noticed the hobby plan details say that it is $7/month/dyno. This confuses me a bit, is there any way I can be charged more than $7/month with one application running on a Hobby plan? If a ton of users happen to go to the site will there be some Dyno scaling that will end up costing me more than the list price? Is this possible? Thanks