Great comments above. Wheel building is a craft that must be developed over time, not something you can jump right into and call yourself a pro.
Realistically, with all the ready to ride wheels hanging from hooks in bike shops, or being sold on line, the custom build market is quite limited,
However, there is a market, and probably always will be, so if it's an ambition you have, I would encourage you to pursue it. Build as many wheels as you can for as many people as you can, even if it's just to cover the price of the parts.
When you have a list of successes behind you, and people who will vouch for you, approach some bike shop managers and ask for work.
Read everything you can by Jobst Brandt, Gerd schraner,and follow related threads on this forum. Pay particular attention to responses by Mrrabbit and FBinNY and you will learn much.
After you gain some experience in the field, you might consider signing up for a DT Swiss certification course.
Advanced Certification Seminars
I have not taken this course, but even after having built hundreds (not yet thousands) of wheels, I am considering doing it.
That brings me to my situation. I came to wheel building, and bicycle mechanics in general relatively late in life. I built my first wheels when I was in my forties, and at age fifty, was taught wheel building hands on by John Barnett. That was ten years ago, and the learning and developing of the craft continues.
I now build wheels as part of a multiple income semi retirement lifestyle. I don't think I could rely on it as a sole source of income.
I am the in house builder for a high end local road bike shop, building their store branded wheel sets that retail between $1,200 and $1,600.
My hat is off to the guy that build 8 to 10 wheels a day. I think my record is 6. However, there is no way I can dedicate full days to it, so maybe I could hit those numbers if I had nothing else to do.
Just checked my records, and my year to date count is 58 wheels, 54 of those for the bike shop.