Limiting your tests to using one assertion is a controversial topic. I originally stumbled upon the idea on Dave Astels’ blog. I liked the style of development that Dave described and decided to give it a try, that was over 2 years ago. Since then I’ve worked on teams ranging from 4 developers to 16, codebases in Ruby and C#, and project timelines ranging from 3 months to 8. I think it’s fair to say I’ve given the concept plenty of chances to fall down. But, regardless of the variables, the guideline has always remained valuable.
For me, the main motivator for using one assertion per test is the resulting maintainability of the test. Tests that focus on one behavior of the system are almost always easier to write and to comprehend at a later date.