Hi, you can check the number of data records in a data unit using System Diagnostics (OSD) from the solution exchange. Create an app snapshot and then check the report that shows the largest data units in the app.
As I understand you are still in development, so at this stage, the number of data units can only be based on an (educated) estimate. E.g. how much data do you intend to load per period for a given scenario into the entity (provided you use the entity dimension for entities)? Or get some test extract from your ERP and perform some test loads using the entities that are likely to hold most data records. Also, do not forget that loaded data just gives you a rough first indication of the number in your data units. The data is calculated, allocated, transformed, aggregated etc., which can multiply the number of records in a data unit. And, extensibility should be applied in a reasonable way to ensure that the top members in the entity dimension do not end up with massive data units.
Remember, the data unit is defined by the combination of the following dimensions: Cube, Parent, Entity, Scenario, Consolidation and Time.
Your implementation partner should be able to help you with this.