Data Robotics has some cool calculators. Basically it's N-1 if all drives are the same size. If multiple sized drives are used, it is the sum of all drives minus the size of the largest drive.
Check out http://www.drobo.com/resources/drobolator.php for more configurations.
Right, what I meant was, how does it indicate the available space left... I understand that if you have two 2TB drives, you have 2TB of usable space. What I meant was, say I set drobo to show Windows that it's a 16TB drive (for expandability in the future), how does Drobo show me how much actual space I have left for data? I need more than lights flashing a certain color.