I think the best way to understand is not necessarily unit count but, what is the level of difficulty or the needs for the assets? Are the assets stabilized? If so, how will the next 2 years fare for these properties? Are there specific upcoming challenges or needs such as rehab programs? Refinancing, capital improvements, etc.?
If no real plans other than daily operations, you're probably fine without a RM until growth occurs with 6 + properties and/or 1,500-1,800 units.