There are a number of things to consider with your business unit set up such as using the connector to synchronize data between SalesCloud/Service Cloud and if you will need a Multi-Org setup.
Support may be recommending an additional 2 child business units (BU) as it is good practice to keep the parent business unit as the top-level admin BU — especially if you need to keep subscriber data separate in your child level BU. For instance, if you want only subscribers who opt out of Child BU 1 to only opt out of that BU and not affect subscribers in Child BU 2, then you will need 2 additional BUs.
Also keep in mind that the All Subscribers list on your parent BU will hold all subscribers in your Child BUs. Therefore, it would be extremely difficult to keep the data separate in just 1 parent BU and 1 child BU.