A project I'm working on now is up to 57 model classes and is still growing. That's a lot of classes - welcome to domain modeling. In my opinion, the number of classes is a fair tradeoff that keeps each class simple enough to understand. In some ways it moves complexity out of the model class internals into the inheritance hierarchy, which is an important part of object-oriented design. I've worked on projects with many more model classes than that too. (Financial applications seem to require a lot of classes to model the complicated workflow and permission systems.)
The place where it starts to get hard to manage is when I look at the file system and see so many files in one directory. My brain usually starts to overload when I see more than a dozen or so classes in a directory. My first inclination is to throw some related class files into a subdirectory. The problem is that the standard way to do that in Rails is to put those models classes in a namespace (module). Rails used to have big problems with namespaced models, mainly with the dependency auto-loading code that finds class files based on the model class name. Most of those problems have been fixed, but there are still some usability issues with namespaced models.
