What is the least number of clothing items you have run on a washing machine?

I will do one item on delicate if the machine has a small or mini-load feature.

Zero.
I put them inside

Maybe one pair of dark jeans if it has really heavy dye or if it smells like a pesticide. It would have to be something I would have a hard time hand washing and wringing out.
But underwear? I would wash that by hand by itself. I wash all my new clothing by itself first. I have learned that labels don’t always mean squat. It can say colorfast tested and still leak. So I like to wash new items alone. Of course the smaller the item the more likely I am to hand wash it. Jeans are just too difficult and you waste more water trying to rinse them out over and over again in the tub.
But many clothing items are either made with material that has pesticides in them or have been put in storage places with pesticides and should always be washed first alone. You don’t want to just spread the stuff all over your other clothes.

If I were washing new underwear, I would throw other dirty items in with it. The least I have washed is one, but not underwear, I have washed one sweater or one jacket. If just a little tiny piece of underwear, it definitely would have other things in with it. When you think about the energy and the water, it’s ridiculous to run that for just one piece of underwear.

We don’t know what the underwear item was. I might let certain items slide.

I have washed one item on its own. It might be a black top that I don’t want to pick up any lint. Or something that was particularly dirty and I didn’t want to put anything else in with it. Or something I know is going to shed/run quite a bit. Not underwear though. I would hand wash underwear or wait until I have a larger load that can go in with that one piece.

One. On the smallest amount of water. It was something I was afraid would bleed color and didn’t want to take a chance.

One also. When you need it washed, you need it washed.

Everyone has a favorite pair of underwear that they can’t live without (or maybe just one pair of underwear).

Yes, one could wash a few more things to help with the water usage, but your friend shouldn’t get his undies in a bundle or he will need to wash em too!!!

Like @ragingloli it’s zero, but for a different reason. To wash my high-efficiency washer, we occassionally run it empty.

As for clothing or items to launder, several times a year I wash one item by itself. Due either to needing it asap, the item being delicate, or an item being too dirty to wash with other items.

One, but only on very rare occasions. (i.e. if I am treating a stain on a particular thing, have something especially delicate to wash, have brand new dark jeans, or I only need a specific item washed and don’t have enough for a load.) Other than that, I will almost always wait until I have enough clothes for a medium/small load at the very least. Routinely washing so few clothes is wasteful and inefficient.

One or two pieces of clothing that were very dirty and I didn’t want their filth (usually spilled fuel or cow poop on the clothes when my husband was working at a ranch) being transferred to our usual dirty clothes.

If I had just a pair of undies to wash, I would wash them by hand. I have done this many times before. I can’t see wasting water and energy by using .a washing machine for such a small item of clothing.

I try to put in as many as it will hold, but I have been forced to put in as few as four when my Grandson gets soiled and I have nothing to change him into.

I send both of my grandsons fully clothed, but they often come to me with nothing on but diapers, and my supply of clothing gets depleted every month.

Fuck that, I hate doing laundry, if I have to do it, I’m going to wash as many items possible in that one load.

I never less than half fill it. Would something on its own wash properly?

I have a throw rug that I wash alone. If I wash anything with it, the “fibers” from the rug just stick to it and ruin it.

It needs to be at least half full because the lowest water setting I have half fills the machine. When there is extra room I will search all over the house to find items to put in. I’ve never washed just one or two or three items. I hate waste >.<

Depends on the size of the garments. I have a washable coat that I wash on its own because nothing else will fit in there with it. I don’t run the machine less than half full though. There’s always SOMETHING else I can stick in there.