I wouldn't lump all small solo mental health practices in that loop.. most of the small ones i had supplemented their income with other means of revenue such as nursing home visits, prison visits, court testimony's, forensic evaluations, and more..
while the psychiatrists could easily rake in a nice living doing all med managements and eval's without breaking a sweat..LOL
I spent a lot more than 2 hours on my psychologists, actually they were a lot of work. I had a solo pediatrician who saw about 15-20 patient's a day and worked 6 days a week plus hospital rounds, he kept me pretty busy too.
I just don't think you can really determine how many accounts one person can handle at a time, there are just too many variables. I know a billing company who like me planned to keep going w/out hiring and she handled 40 accounts (chiro, and pt) all by herself, but she also worked 7 days a week, took no vacations and was up till the wee hours of the morning.
She didn't mind..she still doesn't. She has just adapted and enjoys it, she's not taken on any new accounts in a few years but now also does seminars in the mid west ... my point is that you choose to grow or not and handle things as you go.
You should have some sort of goal in mind about growing your business. It's also better to be prepared in other ways as well. I know billing companies that start out with the cheap software and later on learn the hard way it was better to start off with a good one rather than lose time with a conversion (big headache.. I did it a few times)