The Facebook account is what you use to login to Facebook, inside your Facebook account you can have an unlimited amount of ad accounts.
So let’s say you login to 1 Facebook account but have 5 ad accounts in your account. This means you only need the 1 account since you only have 1 Facebook account. The number of ad accounts does not matter.
However, if you want to give unique access to TCM for your clients or team members then you would need to purchase additional Facebook account access for each user you want.