Everything I read online basically said for such a small amount that 1 teaspoon of salt would be fine so I went with that. I did this in a 12oz mason jar and I just dissolved the salt into 1 cup of water and poured it over once I packed the onions in the jar. It was definitely a good amount as these were bubbling like crazy when I checked them the next day!
If I do these again I may use red onions since that is what almost every recipe I found called for, I just happened to have white onions on hand at the time
I made a post a few weeks ago about my pickled onion/habanero mix! I already used up the whole jar lol