Most churches with multiple campuses put them all into one Planning Center account. Some Planning Center applications allow you to organize them using folders. This allows you to create folders for each campus to keep them separate, yet have the ability to access people from multiple campuses.
Setting up one account with multiple campuses gives you one bill for each campus. You also get a couple of other campus features detailed here. Your account only gets one name and one logo, so if your campuses behave more like different churches, you should create multiple accounts.
Users that belong to multiple churches can link their accounts together to switch between them easily and sync their contact information.