I've just noticed that this question converted to Community Wiki after 16 answers had been posted.
I thought that the threshold was after the 30th answer had been posted.
As detailed in the answer to this question on Meta Stack Overflow, the threshold can be set on a site by site basis and is now 15 for Programmers (along with Super User, where the threshold was originally reduced in an effort to stem the tide of pile-on answers).
It doesn't affect existing questions, but if a new answer is posted to those they will convert.
As of April 9th 2014, all of the automatic triggers that convert a post to community wiki have been removed.
This includes the trigger for the nth answer (which was site dependent) and the mth edit to a post.
The corresponding blog post announcing these changes is Putting the Community back in Wiki.