Sometime one class per file, but...
When multiple classes are stricly related, more than one class in the same source file is, IMHO, BETTER than dedicating a short source file to each class. The source is more readable and compact (and using #region the same source can be be more structured than before).
Consider also that sometimes it's NECESSARY to spread the same class across different files (using partial), since having a 20000+ line source file is not handy even with the RAM I have available (but this is another question).
