Hello Mchernan, and thanks for the great question about adding more domains for hosting.
You're able to upgrade your hosting plan in AMP so that you can host extra domains on your account.
When you go to change your plan in AMP, it will show you the price difference and if you have any credit on your account to use for the upgrade difference as well.
If you currently have a limit of 2 separate domains, then you'd be on the Launchbusiness class hosting plan. You could upgrade to the Power plan which allows for up to 6 separate websites on the account.
If you had any other questions at all, please let us know!
- Jacob