How is your rental business structured at different levels of units??

[BOB CRANEY]

I am a small LL with 13 rental units in 9 buildings. Most all i have is row homes or duplexes with 2 units in one building. I manage and do most all my own maintenance with the exception of most of the major stuff. I have contracting experience so anything i cant do myself, i have someone to call to fix it. I would like to build up to at least 25 units so i can replace my regular Joe job income of about $90k per year. 

I am not stuck on that 25 unit # but wonder how much more business structure i would need to manage 25-50 units whether they are SF, row homes or small multi unit apartment buildings. Curious to hear how other people are set up, # of employees and what they do vs what you still do and are responsible for daily. I find 13 units very easy to manage currently but realize that there is a line somewhere that requires more help if you go up in unit count. Currently grossing about $174k per year or about $13,385 per unit/year