There is no right answer to this one. It all depends on your availability and how comfortable you are with training various types of clients. You are in this business to make money, so I think the more clients you can handle the better for your income. Everyone has a breaking point (burnout), so unless you already know yours there is only one way to find out. When I was training out of a gym, I was averaging 10 clients a day, six days a week. That lasted for a few years until I started my own company and became mobile where due to travel time I’m now averaging 6-7 clients/day. For you that number might be different. My suggestion for you, is to find a balance between your professional and personal time, so you can enjoy life while still making some good money.
this question rarely presents itself because new trainers usually do not have a line of people anxiously waiting to train with them. I also do not think that there is a number that one can give.
For me, seeing 5 or 6 clients makes for a full day that is still balanced with other activities I like to engage in.