I have to disagree with math-ya's comments. A more highly leveraged investor with minimal cash flow and many units can ill-afford a property manager. Not only will that person pay the management fee, he or she will pay for all the stupid mistakes and miscommunications of the property manager that add to the operating costs for his or her properties. That slippage can add up to another percent or two in expenses, even if you manage the managers. Property management companies take on too much and try to streamline. That results in things being replaced instead of repaired, the water being left off or on, damaging the landscaping, pipes breaking in vacant buildings, and the like. If you are running thin margins, you need to be looking at every dollar before it is spent, not after. Trim the portfolio, fatten your margins, and go on vacation because the best property manager you can find is doing a reasonably good job.
I have to hire property management for the houses that are in another state and 700 miles away. Way cheaper than hopping on Southwest every time there's a problem that can't be handled by telephone or you need to find a new tenant.
I do agree it's very easy to make acquisition mistakes in trying to grow too quickly. Did that myself a couple of times, and had to learn the expensive way.