Holdouts are Turing machines whose halting status is not yet proven. While a final proof of BB(6) is potentially far in the future, the number of remaining holdouts can provide a measure of progress towards determining BB(6) and the machine(s) that achieve it.

The result will be taken from official statements from the bbchallenge.org site, if available and the site and its associated discord and wiki are not defunct. For example, as of 2024-05-02, there were 2833 holdouts for BB(5). There is not an official holdout count for BB(6) as of this question writing; informal announcements suggest the number is approximately 12,000.

If the bbchallenge site is defunct, or there is no official count by the close date, I will attempt to determine an alternate resolution source.