I haven't had any personal experience with Phoenix Worms, but I do feed my bearded dragon Reptiworms. And I don't think Phoenix worms and Reptiworms are much different in terms of satiety-effects, so I'll put in my two cents:
A baby bearded dragon (which I'm assuming yours is, based on the length) can go through quite a lot of feeders in one feeding. My bearded dragon probably isn't the same age as yours (but it certainly isn't the same length as yours), but he's still between the age identifiers of "baby" and "juvenile." He's always eaten close to 50 feeders per feeding, which last ten minutes, two feedings a day, whether it's crickets or Reptiworms. Young bearded dragons will definitely go through a lot, so I would expect you to go through quite a few Phoenix Worms within the two to three weeks that they can survive.
My bearded dragon goes through 700 (give or take) small Reptiworms in about a week (about 100 a day). But all bearded dragons have different eating habits, eating speeds, and preferences in terms of feeders, and my bearded dragon's age and length is quite different from yours (he's a little over three months old now, and is a foot in length, counting the tail).
I would recommend buying just a few of them at first to see if he even has an appetite for them, find out how much he'll eat within 10-15 minutes, and then if you plan on buying more, base your purchase on however many he'd theoretically go through based on your calculations over two or three weeks.