This was a feature of 8cars, but the count was never consistent or accurate, drove me kind of nuts! So I ran a handful of maxed out parks through Open. The "show_limits" for the object count was nearly bang on! This will make building with the object limit so much easier knowing the exact space left... until the object limit is removed of course.
If you have a map of (what OpenRCT2 considers) 254x254, you have 256 x 256 surface elements, or 65536. This includes the outer ring of invisible surface elements that are there to have the map edges appear correctly.
A request to people having trouble with pathfinding. I'd like to have parks that currently have pathfinding issues in OpenRCT2, so we can test pathfinding improvements. If you submit one, please also tell/mark where the pathfinding goes wrong. If you don't want to make your park public yet, you can send it via PM and I won't share it with others.