I have never been successful at counting out the number of songs I know, my brain clearly has an inefficient filing system. The hardest part I have with remembering songs is getting my brain to think past the last five that I've learnt. I find myself always singing the same few songs over and over every day and start losing others that are deeper back. Now and again I like to write out a list of songs I think I know and then force myself to sing them in any order so that I go back over ones I haven't sung for a year and try to keep them current. At the moment I'm engaged, in odd moments, in trying to write out the words to all the songs that I know and can write down from memory into a small book. Its not full yet, 25 songs, but I haven't been working on it for long either.