That is what I read previously, that would mean you should stop at 39/40. However, what I don't understand is that my mine (and as I have seen on some of my neighbors also) is still open and allowing expansion, and I am on 3/57 currently....I'll attached photos.
I know one of the Mods will prolly move this post to the correct forum, but I've been trying to figure this out for a while, so I am asking while I have a chance to get some answers. Sorry!