I keep one pair of sunglasses in each vehicle. I also like to have a pair and a spare of regular glasses. I only bought so many at one time because my prescription changed. And I don't mind buying multiple pairs, because I paid for 4 pr what a lot of people pay for one.I had one pair of glasses for about 15 years, didn't wear them much but they helped some at the range and for night driving. Then when I was about to retire I bought 4 various pairs because I thought my glasses benefit was going away. Turns out it didn't, I still get reimbursed for glasses. I have barely worn those glasses but now I think my prescription has changed and I probably will have to get a new pair anyhow.
I thought buying 4 at one time was very excessive but then I found out lots of people do that.