What about musicians that often play from sheet music, and they only have to play the same tune over and over!

I use a book containing first lines of verses, as an aid to remembering - but I do know around 300 songs, and as I get older (now 63) I find it more difficult to learn songs, but there are some I have known for 40 years but seldom sing and always remember.
I would however, NEVER do a paid gig singing from my notes - but I will certainly put the songs first lines on my gig list - and Practice before the gig until I am tired of the songs!!
Tim Radford