My personal experience with Harley is DON'T feed them as much as they want of any insect. I did that with Harley and let her eat her hearts content on hornworms, dubias, silkworms, phoenix worms and butterworms as she was growing up and now she is 17 inches(yeah I know that's a bit small but I've had her since she was a month old and always fed her 3 times a day as much as she wanted and had her set up just right) and at last weigh in at the vet just over a week ago, she weighed 695g. She holds out for bugs all the time and getting her to eat salad is like pulling teeth. Luckily the vet said her blood work came back fine except a little high protein level but that was probably all the bugs and that she was overweight. She is a sweetie though and I just cave in when she cocks her head side to side staring at the salad and then looking at me like "This stuff is for you! Give me my sweet sweet bugs!"
As for how many, I'd say for an adult dragon, probably around 5-6 roaches around the width between her eyes a day.
If you are planning on getting 2 babies, I would definitely start a colony now. Harley ate around 50-60 roaches a day as a baby. It was expensive keeping her fed til I started my colony. So multiply that by 2 and yeah you get the idea. Plus you can always sell off the surplus on Craigslist or something. I usually make enough a month that the roaches more or less pay for themselves.