Some years ago, I thought I wanted to scale from a solo freelancer into an agency. I had two reasons: 1) money and 2) accomplishment. It's now my belief that building an agency is not a good way to achieve either of those two goals.
I went a small ways down the path of building an agency. I worked on a project where I hired two other freelance developers and one designer to help me. It was a lot of mental overhead and at the end of it, I didn't make that much money. Taking into account the stress of managing other people and the risk of being responsible for other people's work (including mistakes), I ended up wishing I hadn't gotten any other people involved at all.
One of the least attractive parts of running an agency I can think of is the boom-and-bust cycle. Almost every freelancer/consultant has a boom-bust cycle. With an agency the symptoms of the boom-bust cycle are amplified and made much more painful and serious. I've seen agencies have to lay people off when times are lean. Then when times are "good", you have to scramble to hire more people to do all the work - or make your employees work overtime, which I think is pretty unfair (I've been in this position at agencies).
So at some point I decided I didn't want to build an agency after all. Instead I decided to make as much money as possible (in as few hours as possible) as a solo consultant. Based on the fact that [Alan Weiss](https://www.amazon.com/Million-Dollar-Consulting-Alan-Weiss/...) claims (in such a way that I believe) to earn over a million dollars a year as a solo consultant, I understand it's possible to earn quite a lot as just a single individual.
My biggest success in increasing my earnings has been to move from development to training. I haven't succeeded in charging more than $100/hr as a developer (except some tiny projects and emergencies) but I've charged as much as $13,000 a week as a trainer. Some trainers I know charge $20,000 a week or more.
I've also had some small successes selling products. I wrote an ebook in 2016 that sold about $8,000 worth. Now I'm working on the next product.
So if someone came to me with this question of how to scale into an agency, my answer would be that I think there are way better ways to get what you think an agency will get you, unless for some reason you really want to start an agency for the sake of starting an agency.