For a multifamily investment, how many units are desirable for an onsite property manager giving up one unit?

If I understand the question, you are asking when is it profitable to have an on-site property manager. If so, I don't believe you can base it on # of units, alone without taking into consideration the overall net from operations (NOI). In order to have an on-site manager it would need to cash flow to meet your intended expectations. With that being said, there are many property management companies available that can service your investment properties without being on-site very effectively and much more cost effectively to you.

Hello Chris, I agree with Janine and her comment that it is mostly matter of numbers but from my experience I would use the following: Under 20 units no on-site management, up to 50 units on-site engineer perhaps only part time for after hour emergencies for partly subsidized rent, over 50 units on-site property manager.

Agreed. Janine says it best. In my market (north of Boston, MA) property managers are getting anywhere between 5 & 10 percent of the gross monthly rents. While that expense might be higher than a "Super", your return on the investment is greater in other areas such as number of people who are responsible for your property and their level of expertise with repair & maintenance.
Brian Dapice
Keller Williams Commercial