While there is no hard limit for the amount of comments that can be posted to a thread, Disqus intentionally slows down updates on threads that grow larger than 1,000 comments. While the discussion will still load on the page, you may notice that the thread is slow to update with new comments, votes, deletions, etc. This helps Disqus perform better across our entire network and prevents large threads from straining shared resources.

The best practice is to "archive" and close a large thread so that you can initiate a new empty thread in it's place. This will guarantee your community a better experience while using Disqus.

Additionally, overly large threads can be avoided by automatically closing them.

To load a new empty thread in an existing page on your site, you'll need to simultaneously update both disqus_url and disqus_identifier with two new values in your embed code. This will effectively replace the existing thread on the page with a new empty thread. More about these variables can be found in javascript configuration variables in the embed code.
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Below is an example of how you would change the configuration variables for an existing large thread:

Note: Both configuration variables must be changed. Updating the disqus_identifier variable without changing the disqus_url variable will only append a new identifier to an existing thread and will not result in a new empty thread on the page.

The disqus_url value will be used as the URL for the thread in Disqus, so you'll need to define a value that your site handles correctly. You may need to use parameters in the URL, for example: http://example.com/post/cats?v=2.

After creating the new thread on the existing page, you'll want to close the old thread to ensure that people stop posting to it. It can be helpful to your users to include a link to the new thread as a featured comment in the old thread.