When you're in the account section of Middle, you'll see a list of all your accounts. Middle is built for multi-tenancy. This means you can have multiple businesses, clients, or brands in separate workspaces all within the same Middle instance.
Owners of instances can view all accounts. If you want to provide limited access on an account-by-account basis, this can be managed through account-level user settings.
There is no limit to the number of accounts an instance can have.