There is no limit to the number of classes you can have in a CSS file. My guess is that you had left off a bracket in your previous css class definition, or had some other typo that made your class definition fail.
My first go-to for diagnosis is Chrome's inspector. Right click the element on the page you are trying to select and view the results in the inspector. You should see your class definition, if not double check to make sure your names match between your html and css file. Secondly, I would check to make sure that the class definition that proceeds the faulty definition has been closed properly. You may be missing the closing bracket '}'.
If you do see the class definition in inspector, but your css changes have not been applied, you should see you attributes striked-out in the inspector. Follow the chain of classes beneath to see if you have some other styles that have precedence over your class.