Multiple projects are recommended, and are necessary to get the full benefit of CSLA and its n-tier architecture.
If you know beyond the shadow of any doubt that you will never need n-tier deployment, never need advanced security, never need increased scalability, and generally aren't looking to build an enterprise level app, then you can use fewer projects.
In fact, if you are building an app that you know you'll deploy as a 2-tier app then you can put the business classes and data access classes directly into your UI project along with the UI code.
Is that a good idea? No.
Will it work? Yes.