Ghostwriting is a craft business with a brutal scaling problem.

Your product is content that sounds like a specific person. Producing that requires deep familiarity with their voice. And deep familiarity takes weeks.
Most ghostwriters handle 3-5 clients well. Quality is high, retention is strong. Then someone offers a sixth retainer and the math breaks. Taking it means spreading thinner.. which means voice drift.. which means churn.
So you say no to growth.
The bottleneck isn't skill. Human voice-learning doesn't scale linearly. Your bandwidth to absorb and replicate a new writing style tops out somewhere around 5 active retainers.
What changes with per-client voice models:
Onboarding from 2-3 weeks to 5 minutes. Editing from 80% instead of writing from zero. Revision rounds drop from 3 to 1. Active book moves from 3-5 to 8-12 clients.
When a client says "this doesn't sound like me," you point to a 92% match score and the specific signal it missed. Subjective taste becomes data-backed review.
Full breakdown of the workflow in the link.