deployments are immutable; each deployment has a permalink that will always work for it, until you delete the site. This screenshot shows how to get there from your deploys listing:
These URL’s look like https://DEPLOYID--sitename.netlify.com, and deploy ID’s are unique and specific to that very commit or deploy.
The “deploy preview” URL however, - something like https://deploy-preview-1234--sitename.netlify.com reflects the last successful deploy built off the PR, but the first one you made will be there forever, or until you delete the site. That’s the one you can “zero out” with an empty deploy. But the previous deploys live on at their unique URL’s.
Deploy previews automatically get marked as non-crawlable so there is no problem with having thousands around. Some customers have built over 20k times on the same site; things work well for them.
All bandwidth that we send to web browsers for your website counts towards your bandwidth bill. Nobody will “find” those old deploys unless you share the URL. Nobody has ever been “attacked” on a deploy preview URL to cause bandwidth overage before, so no need to be worried 
All builds count towards your build minutes bill. Don’t make needless builds; read this article for details on how to configure things:
You can see those running totals in your team dashboard.
- Bandwidth here: Netlify App
- build minutes here: Netlify App
There is no “master list” of all deploy preview URL’s. You can see all your sites in the site listing page:
Those update about once a day; not yet in real time though we are working to bridge the gap.