Like the aging druggie, when asked just how many drugs HAD he taken, after a long, thoughtful pause said...."LOTS." I've been singing folk songs for about 40 years, and 30's-40's tunes for longer than that (altho rarely, with my folkie chords, can I accompany myself). I have no idea how many songs I know, but I'll bet half of them I'd not be very interested in singing any more.
Just because I know tom Dooley doesn't mean I want to sing it any more! I do find it harder to memorize, these days, the only way I really like to sing...so like Splott Man, I've been making a conscious effort to learn 2-3 new ones/month.