Maybe “always have on hand” is a better way of putting it than stockpile. What stuff do you always have backups of so that when you run out/use up <whatever> you have the replacement(s) already in the house?
For example, I have a countertop water filter system [this, and I highly endorse it] that takes a big filter, which lasts about a year. As soon as the device signals me that the filter needs to be replaced, I replace it with the one I have on hand and promptly order another one.
I always have batteries-- AA and AAA. Even though most of my stuff is rechargeable (which has led to cords being plugged in all over, and a nightly ritual of plugging in phone, kindle, bluetooth, etc., for the next day), some things still need batteries, and I ALWAYS have them on hand. I buy 60 hearing aid batteries at a time from amazon, and always have spares with me. They last about 10 days. The hearing aid gives me a little arpeggio when the battery has 15 minutes of life left, and it’s seriously inconvenient not to have replacements handy.
I hung party lights in my back yard when I moved in here a couple of years ago. There are three strings of 50 lights, and when I ordered them, I bought two boxes of replacement bulbs. They aren’t the sort of bulbs you can find any old place.
Speaking of bulbs, I always have light bulbs on hand, too. I buy the Chromalux full spectrum bulbs from amazon a dozen at a time. They last forever-- a couple of years-- but when my stock gets low, I order more.
If I find pens I like, I buy a box of them.
I’m definitely not this way about everything. I guess the stuff I stockpile is the stuff that when I run out, I’m going to get really pissed off, and the replacement might be hard to come by.
I’m not specifically asking about stockpiling food, but if that’s one of your things, feel free to include food.