When I was a newbie, my boss told me that I should never answer the question "How many clients do you have." See, it's a question with no right answer, in which the asker can interpret the answer any way they want.
If I say a high number, does that make me sound Healthily Busy or Totally Overwhelmed? If I say a low number, am I Selective, or Lazy? And what would BE a "low" or "high" number, anyway?
Since I'm no longer a new agent, and I believe in transparency, and I love love LOVE talking about my authors - I have a list of my clients posted on my blog. You can count them if you are so inclined. (Spoiler: it's about 50). I still shy away from saying an exact number out loud, and I don't have a number that would be a "ceiling" in mind, but, you know, basically I am pretty full. I am busy enough. I don't need more clients. But I shall certainly leap to grab one if the perfect fit comes along!
I don't remember where I got this analogy but I think it's a good one: it's kinda like being an obstetrician. While I might have dozens of clients, most of them are busy
gestating or taking care of books under contract that already exist -
they aren't in my waiting room with their water breaking all at the same
time. This metaphor has gotten a little gross now, but you see what I mean - there are only a few pressing matters "in play" on any given day/week.
So how many authors IS too many? I think of my list kinda like Mary Poppins's bag crossed with the TARDIS. Magical, flexible, sentient, bigger on the inside, and obviously able to navigate wibbly-wobbly timey-wimey stuff. There's no answer, is what I'm saying -- how much work I can do adjusts according to how much work there is to do.
If there is ever a time I am feeling burnt out or overwhelmed - I take a break and/or ask for help. If I was feeling like that all the time, I'd cut back my list -- but so far, luckily, that has not had to happen. Other agents' answers may vary -- for some, 20 clients would be their limit. Some, I'm sure, juggle 100. But I think everyone would agree that there is no magic number, and no right answer to this question.