Q: What is the best way to use classes to track several rental apartments in one building?
A: Honestly, I would NOT suggest using classes to track units at all. If you manage multiple properties, each with multiple units, the class list would become cumbersome to work with.
I would add each unit as a Job of the property. This will keep your property owners and tenants organized in the Customer Center.
Add each Unit as a Job. Open the Customer Center. Right-click on the property and select Add Job.
As you add property owners, properties, and even tenants, adding the units or apartments as a Job will keep your financial and management data organized. The screen below shows how your Customer List should look after adding property owners, properties and units.