If I remember correctly from one of my Imagineering Field Guides, They use a formula called THRC (Theoretical Hourly Ride Capacity) to calc. this stuff. It has to do with the interval between ride vehicles bein' sent out, how many Guests the vehicle can accommodate (as mentioned above), etc. From what I understand, first and foremost is choosing the proper ride vehicle to tell the story, and then trying to figure out how to maximize THRC. Sometimes, the right vehicle to tell the story isn't the highest Guest -mover, but the popularity of the attraction makes up for that.
Like everything else, it all boils down to the numbers, but there also has to be a balance with the ability to tell the story, from an Imagineers point of view, properly.