This depends on who your legal customer is.

Single customer / one contract for all units:

Each unit is simply regarded like any other room within a building.

• If all units are managed under one contract and you do not contract directly with each unit occupant, create one Building record.
• Enter and document equipment within that Building, making sure to set the correct Location and Area Served for each piece of equipment.
  
• You may enter multiple addresses for the same Building by adding them under the Building’s Extra Address tab.
  

Separate customer per unit:

If you contract directly with a unit owner, treat that unit as its own customer relationship with their own building.

• Create the unit owner as a separate Organization
• Create the unit as a separate Building
• Set up a separate Contract.
  
• You may fill in the Parent Building field and a Unit number field accordingly.
  

View this tutorial for more details on entering customers.