Cost is more of a consideration than spoilage
If you buy in bulk, most of the big names use vacuum packaging. This will easily extend the life out to 6-12 month without a problem. I place my order once a quarter and have never experienced a problem with spoilage or the items being refused.
As far as quantity, if you are only ordering for one snake, you are not going to get a good price break after paying shipping costs (~$40). If you ordered 50 pinkies the total cost per item would be close to $1.25 - $1.40. A better option would be to find a nearby breeder or herp society and go in with them on an order as split the shipping costs.
If you need to figure out how long it will take to go through 50 pinkies with one snake, take a "worst case scenario":
3 pinkies X 2 feedings per week = 6 pinkies/week
(50 pinkies/bag) / (6 pinkies/week) = >8 weeks
So..........One bag of 50 would feed one snake for at least 8 weeks