In the intervening months since the feature was introduced, there have always been at most 5 comments shown under a post when the page loads. However, it seems that, on Meta, that limit has been tripled.
There doesn't yet seem to be a user preference to tweak this number, so I think that this is likely a bug somewhere.
As an example, check out this post. Instead of showing 5, now 15 comments are displayed under the post, taking up significant space: