How many projects I need for my application?

Ideally, all softwares need one project which will be part of one solution. If you are developing a small application, just create a new project and you are ready to proceed.
However, large softwares will need several projects. All web pages will be organized into one project. All the business layer classes will be another project and data layer classes will be a different projects. Some of the utility classes may be organized into another project. Typically, a large software may have 10 to 20 projects part of the solution. Large softwares may even have several solutions.