If you have less than 8 spirits, you can add new ones up to that number. You can change any number of spirits that you already have into something else. Your character can possess any number of spirits between 1 and 8, you don't have to maximize your number of spirits. Check Olivia's character sheet for example, I only have 2 spirits there at the moment, which are 'new' or different from what she had before. So, you can also remove existing spirits.