Oh, sheesh. I don't ever donate them, I just keep pairs I had, even from the 80's. That way, I can use some old ones when I'm out painting or doing a dirty job, and keep my latest ones clean and nice.
Let me think: Two current ones, a regular and a reading glasses.
And a pair of pink hippie prescription shades from the 90's. A pair of big round frames from the late 70's. And maybe one other pair. So a total of five.
My prescription never changes that much, so I can still use any of them.