What is the minimum amount of units (in multi-family apartment) where an onsite property manager is not required?

[Gautam Shah]

The question is in regards to balancing property management costs.  

I've noted, that generally apartment complexes require a property management company that takes between 5-8% of gross rents, and also hires payroll staff to be on site.

Eg a site manager, and an assistant to a site manager, and a contractor for 50+ units. In evitably, this results in reduced NOI, which in turn reduces the cap rates and cash on cash returns for investors.

If one is seeking to pick up another apartment, then what is the threshold level for # of units one can own without having to employ an onsite property manager (but still retain a property management company to handle the leasing, maintenance, etc)?

Is this strategy advisable? Why or why not?