If you have more than four units, you will need a property management company, or at least an onsite full time manager. Some local/state laws even require it. Make sure and check the laws in the area of the property you are thinking about buying.
Multi family units are a business investment, and you should run it like a business. Please hire someone to do the day to day management for you.
Much Success to you!
Kawain Payne, Realtor