In the interview Craig Federighi gave on Friday, he said 30.[1] I have no idea if he was just throwing a number out or if that's the actual threshold.

[1]: https://tidbits.com/2021/08/13/new-csam-detection-details-em...

Either way, it's high enough that adding a single file to the set wouldn't be a useful way of finding people who have that file. One attack I can imagine would be to add a whole set of closely related photos (e.g. images posted online by a known dissident) to identify the person who took them, and even that would be a stretch.