It's all about how elaborate and far you want to go. For general band stuff (without any or only a little midi) I use 1 project per track/song. For electronical music 1 to 3 projects (depending on the complexity I might have to if my computer can't handle it all in 1 project). For heavy midi stuff (orchestration etc.) 1 project but more often 2 to 5. For film stuff (voice over, sound fx, music etc.) maybe even more.