Now that I have more detail I would say this:
It’s great that you want to concentrate on your training and want to be efficient and do what you do best however just a few things to think about from my experience:
I don’t train every person who contacts me, sometimes it’s not a fit, so I personally like to do a 90 minute evaluation with every client.
Seeing 15 clients a day is intense. I don’t know how old you are or how long you have been training but it would be difficult to stay at that pace and be able to really give each client 100% of your mental attention.
Working more doesn’t always work, you may want to raise your fees instead and concentrate on a realistic work load. There are software programs for payment/reminders/birthday wishes/scheduling etc. People hire personal trainers because they connect with the trainer, it has nothing to do with your assistant.
However, you seem intent on having an assistant which is great. I would suggest sitting down and writing out expectations, sign some sort of contract, discuss pay and hours and then you personally have to figure out how much money you are willing to pay them. Do the math, if it’s going to cost you a few training sessions is it worth it?
How much you pay your assistant would depend on their worth to you.