I think I can do about 150 at the drop of a hat but I do need to go through them occasionally or huge chunks of the words just drop out of my memory.
Learning new ones is getting harder and I'm convinced that as a new one goes into the working memory an old one drops out; maybe I've reached capacity.
I'm re-learning some that I wrote out (but never really learned because others were doing them so well) 30 years ago and I haven't heard for years so I reckon they need reviving.
The annoying songs are the ones you learned but never really liked that much. They just stay and stay and will not go away; cluttering up brain space you need for new, quality songs.