Sadly, currently there is no way to view used resources on an OnDemand instance. If you do end up nearing your limit, you should receive a notification from Atlassian.
If you're running into issues creating a backup of your instance, I suggest that you contact support directly, and they'll be able to assist you further:
Hi team, I’m Avinoam, a product manager on Confluence Cloud, and today I’m really excited to let the Community know that all customers can now try out the new editing experience and see some of the ...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs