Thanks for the advice. Some people do ask out of curiosity in the initial conversation. Or just ask "what else are you working on?"
"I aim for about 25 hours/week of client work so I have plenty of time for professional development and personal projects where I can focus on learning instead of delivering the best results for my clients."
If phrased right, you can make it sound like you're already an expert at what you're doing for them, which is why you don't necessarily learn as much from it, so it's important to spend time honing your skills in new areas so you can expand your offerings.
You could say something like this: "I'm generally not able to disclose details about other projects. But I can tell you that their effect on my availability will be..."
That's only the case if you have literally no other projects. If you have even one other project, it's not the slightest bit misleading. All I recommended saying is that you don't disclose details about your other work. That statement implies nothing about the volume of the other work, except that it's non-zero.