I should say first, for the record, I have an agent who I love. I am completely happy with her, and I have no complaints at all. That being said, she is fairly new (4 years in, now) and has a growing list of clients. I noticed that you have 20 or so clients listed on you sidebar on the blog. I think my agent already has more clients than that.
I know there's probably a balance you strike between further developing your clients careers and signing new clients (vertical versus horizontal development), and I know that I personally want an agent who has time to dedicate to my work, and I am sure most authors would agree. I also understand some clients may only write one (perhaps non-fiction) book, while others may require much more time and effort.
My question is this: at what point (if any), for my friends who are currently seeking representation, should an author be concerned that an agent is signing too many clients? I wonder whether newer agents might sign too many clients (perhaps some who aren't really ready to have an agent) and therefore not have enough time to really represent any of them very well? Should a prospective author even worry about that?
Of course a new author IS going to worry about that. Writers are woodland creatures who worry about every single thing they can think of and when that isn't enough, they look for newly discovered things to worry about. Particularly attractive worry targets are things over which they have no control. This is one of those things.
There is no way to know when an agent has "too many" clients. 15 may be too many for some. 105 may be just right for another. And 105 one year might be 75 the following year. Client lists ebb and flow like every other living thing. I've been as high as 44 and as low as 0 (although that was the year
I decided to come over to the Dark Side, not at any point during my career....so far anyway.)
The only thing you can measure is how your agent works with you. Is she prompt on replies? Does she get work turned around quickly, or at least on a timeline or schedule? Can you talk to her about these things?
I will tell you that I'm so behind on email and reading right now I'd probably fail this test if asked to take it. It's going to get better soon, but like all living things, my inbox and To Do list waxes and wanes.
Worry only about things you can control. Assess a prospective agent's behaviour, and talk to her existing clients. Evaluate based on information received, not worries churned up to distract you from
the plot hole on page 300.