Generally, practices that employ managers earn more than practices that don't. Sometimes that's the result of a great manager. Sometimes it's the result of a great practice.
Here's a look at the percentage of respondents to the 2009 Veterinary Economics State of the Industry Study that say they have a manager. As you can see, among the respondents that don't employ a manager, most say the owner is handling the management responsibilities.
Given that there are 23,957 small animal practices in the United States, we can extrapolate that there are about 11,259 total managers.