Eliezer told me that his preference for new intra-month open threads was 500 comments.

500 is a logical cut-off point because that's as high as the software allows you to display at once without extra loading. Since discussions continue indefinitely on Less Wrong posts, it makes sense to start the new open thread somewhat before 500 comments, so there is some room for discussion to continue on the old open thread without going over 500 comments.