I can't remember where I read it, but it was one of those "unintended consequences things." Being forced to wash clothing by hand meant that people often had only 3-4 sets of clothes. The article/book/source said that nowadays people have many times as many clothes, due in part to cheap clothing, but also because automatic washer reduce the workload of having so many clothes.
45 minutes of fruitless googling got me nowhere. Does anyone have some insight to offer?
There was an article linked to in this old AskHistorians thread, but I can no longer find it online.
You immediately made me think of this article https://www.etsy.com/blog/en/2011/the-history-of-a-cheap-dress/ which is by the author of--well, that article says "The Good Closet," though the by the time the book went to print, its name had changed to "Overdressed."
According to that author, in 1930 the average American woman had 9 outfits. I haven't read the full book, so I can't say if it gives any data for men. It goes on to note that the average American buys more than 60 articles of clothing per year. That article and the book focus on clothing costs, though, not laundry, as an explanation for wardrobe size.
I would say, though, that washing clothing after only one wear is a very modern idea--possibly one that follows from the washing machine--and even still, not universal.