Hi,
There’s no POI limit in the plugin, but your blog can run out of memory. It doesn’t seem likely that’s the problem – 67 isn’t many posts and if you ran out of memory you would probably just see a spinner instead of the map, not an error. Memory issues are covered here:
http://mappresspro.com/mapress-faq
Try deactivating all of your other plugins and switching to a standard theme for a moment, to see if it’s something else.