You can be signed into your ForeFlight account on two iPads (1 main and 1 backup, not to be used simultaneously), one iPhone, and one iPod Touch. If you’re upgrading to a new device, you need to sign out on your old device before you’ll be able to sign-in on the new one. Here are instructions on how to sign out of your account:

If you need to manage the devices hooked to your account, you can do so through ForeFlight Manage. Simply sign in with your ForeFlight User ID (email) and Password, and then select the DEVICES tab on the left side. From there, you can remove unwanted devices by clicking on REMOVE.