Honestly Robert I'm not one to answer this as I don't really even try to sell my embroidery work yet... what I get I take but I don't shop it around. I don't even have signs up really. We are growing and don't want to grow too fast right now. But we are gearing up for a big push forward.
It really would depend on your market. Can you get people in your door. If you think you can get the orders than do the math. You know what an average left chest stitch count is. Factor in that time (I'd say my PR-650 runs at about 450-550 ACTUAL stitches per minute, trims are SLOW) and then you can take a guess at how many pieces on average you will have orders for.
You obviously will have a wall production wise... you can only sew X stitches in a day. So if someone said I need 1000 pcs by the end of the week, it just ain't happening on that single head. But no reason you can't do 100 pcs order in a week's time. Depending what you can sell it for will depend on your profit. If you can consistently average 100 pcs a week and make $10 bucks profit per garment then you are making 1k/week. That's busting a$$ for about 8 hours a day though. This is why a 4 head can crush a single head in larger scale production. Cut your time by 1/4 or if you can sell it, increase your production/profits by 4x.
But all the heads in the world won't do you any good if you don't have the customers.
In the first 6 months I had my PR-650 I made about 2k in profit (after cost of goods/supplies). This was letting the work come to me and occasionally telling on of my computer customers "hey, you know I do embroidery work now". Nothing major, but I really didn't realize how much I was making either... it didn't feel like I was doing that much work. That was about 260 pcs sewn total. That averaged just over $7.50 a piece, but I did 170 caps in that 260.
For me it is a matter of having the equipment to be a one stop shop for some of my customers. I figure that the machine (at about 9k) will pay for itself in about 2 years. That's not great, but then again, it's not really about making a living with this machine. It's supplementing, growing and learning. I don't push this and just take the work as it comes.
I know my post are kind of all over the place but I'm trying to give you as much information and be as realistic and honest with you as possible.
My goal is to have a multi-head soon (auto screen printing first) and then I won't be afraid to take on large jobs. I think I will get a 6 head. But this single head will still see plenty of use. I'd likely run "7 heads" on any high capacity job and load the design on the single head as well as the 6 head. No matter where the thread breaks, I'm still sewing!