In order to keep Unit Tests as simple as possible and easy to understand their intent, a good rule of thumb is to limit the number of assertions to 1 per test method.
This article by Dav Astelsdescribes this approach in more detail.
The main benefit from this approach is that each Unit Test method tests exactly one aspect of the system. If it fails then you (or someone else in the future who must debug the code) should know exactly the pupose of the method and therefore have a better idea of what has failed and where to start looking to resolve the issue.