Without cheat sheets, probably about 20-30 and I can't even remember which songs I do know all the words to. I have a pretty good memory for tunes though and in Hearme for example, I would be pretty confident of getting through most songs I know a tune to regardless of whether I have ever sung the song before.The only problem I have is fitting some of the words in and I usually sing a song through once in my head and make a couple of changes to the words so they fit more easily for me. Sometimes I end up doing this as I am singing but I have come unstuck attempting to do that.
Jon