Let's just do the math.
Let's say you can productively work 8 hours a day, 21 days a month. That's 168 hours total.
Divide that by 55 clients and you get 3 hours a month.
I haven't added in any extra time for your "junior" as he/she is probably already using up some of your 168 hours as it is in managing, mentoring, training time.
There is no way you can achieve anything measurable on 3 hours a month, regardless of what the client has signed up for, regardless of the role and responsibilities you've been assigned. Furthermore, if you flip it around the other way, would you want to accept a client that was only willing to pay you 3 hours a month with the expectation that you could "move the needle"? I know I wouldn't.