These trace elements are to be mixed with N-P-K and Epsom Salt (instructions included), and each 20-oz pack will make up two 5-gallon buckets of garden nutrition. Depending on what you are growing and the number of growing seasons your USDA zone will allow for, you will need approximately one bucket of fertilizer per adult feeding unit per year.
Therefore, if you wish to store up five years of nutrition for four adults, you would need to order five of these 20-oz packs. Additionally, you may wish to purchase a few extra to account for unforeseen circumstances, a change in garden plans or additional mouths you may be required to feed. These nutrients may be stored indefinitely, as they do not go bad.