The general safety guideline for superworms is that your dragon be 16 inches or longer. Some have started a bit earlier, but you don't want your dragon to be too small or there's a much greater risk the chitin in the superworm shell will pose a problem.
As far as how many, there's no firm concensus on that matter. Some suggest limiting a growing dragon to 10 supers a day and using other worms to fill him up, while others say to let him eat as many as he wants once you've slowly introduced them to his diet and you know he's digesting them well. My girl was about 10 months old when I started her on supers and she got around 20 a day at the time. Looking back at it now, I think I should have given her a bit more each day, especially since those were her only live feeder, but its up to you to figure out what you're comfortable with as being healthy for your dragon.