There should be only one bug per report. This sounds obvious, but it's sometimes difficult to differentiate between similar looking bugs and in the rush to get a product out it may seem easier to just lump them together. The problem with reporting more than one bug in a single report is that it's usually only the first bug that gets attention and is fixed the others become forgotten or ignored. It's also impossible to individually track multiple bugs that are listed on the same report (more on this later).