In VS we create a solution.
Then, we can create multiple projects under this. Also, we can add multiple new items under each project.
Suppose requirement is to create a webpart, then how is decision made whether to go for new project in solution. OR new item in an existing project?
You should watch out, that solutions don't get messy or too big. Don't put too many projects inside a solution, it will be impossible to find a webpart then if you have many of them with many projects inside. I always make one solution for like one page, with all the webparts in it inside one project and every "item" of them is in a separate feature. Because if needed, you are able to deactivate not needed ones - one by one.
You can find similar questions Organizing WebParts in Visual Studio - Best Practices and Structure of SharePoint Solution.
When working with SharePoint 2010 and the Visual Studio Tools for SharePoint, you can effectively structure your project how you like - this is because you can use folders to structure things logically
I agree with Chris O'Brien. For example, you have a web part which performs actions on a list, in this case I prefer to include web part to the project with this list.
