I never get just one, because sometimes they will create an ooth even if they haven’t successfully mated. Five to ten is a common grouping amount.
I have had groups with a dud, but mostly they produce.
You have a much better rate of success if you have food ready when they hatch. People claim they are aggressive and cannibalistic, but that must be the fancy exotics. Mine have all been timid about attacking. Right now my new babies are learning to go for fruitfly worms, which are super tiny.
Mantis nymphs won’t go hunting. They find a spot, and wait for something to come along. As they get bigger, they will sort of do rounds of a chosen area, but they still don’t chase, and they don’t wander far.
If you want them to clear garden -pests, I would recommend keeping them indoors until they get the knack for grabbing prey, and also temps have stabilized outside. Cooler than 70° is bad for them. I would scatter them around one or two per plant, maybe more on tomatoes.
If aphids are your main concern, your best soldiers are ladybugs.
If you want just a Luna moth, I don’t have any information specific to them for you. If there are regulations controlling ownership or transport of a critter, that is something to check into first. Some people sell irresponsibly, so buyers should educate themselves before shopping.
There are YouTube videos posted by breeders who sell, and they do tutorials on care and feeding. Be aware there are also dummies who buy, then post without knowing what they are doing, and they give poor advice.
There are some who will sell individual live bugs/arachnids. I would not want to do that. They are more fragile after hatching.
They are good sources for advice, however.