I don’t know how Lisa does it! If I work with more than 4-5 clients/day it’s been a busy day! Besides the workout time, there is the few minutes of greeting, and after the workout the clarification of next workout time and some friendly banter. So, I don’t know how many hours/day you want to work with clients, but I suspect that at 1 hr/session clients, ~6 clients/day is probably a reasonable number.
I like to look at training hours rather than clients because there are instances where I’m working with pairs or small groups. A lot will depend on your business model as well (e.g. are you an in-home trainer with travel time factored in, or do you train in one location where your clients come to you). Because I do a combination of these two models, I feel most comfortable training 6-7 training SESSIONS per day (some days more, some days less).