On the subject of how many songs a singer might need, it would as mentioned by Andy depend upon what you are doing. In an Open mike and singaround situation as he says about 60 would mean that you are not singing the same thing week-in-week-out (It is amazing how many people do this to the inward groans of those who have to listen). If you are performing say three 45 minute sets then you need to have about 13 or 14 songs at performance standard for each set. You will not use them all but you will need to have those at your command.

You cannot perform the same thing week-in-week-out so those three set lists have to change. I remember one group had a spreadsheet where they kept note of what they used to sing, once a song had six ticks beside it it was moved to the bottom of the list and stayed there until it worked its way to the top again. They had something in the order of about 160 songs, we ran on a list of material that was around 240, in a concert/festival environment we always made a point of NOT playing the same set list twice over the course of the festival but you could find yourself playing one set of about an hour, but generally festival slots tend to be shorter.

What also alters the requirement is whether you perform as a solo artist, a duo, or a group and what your normal mix is balance of songs and tunes, mainly tunes with a few songs thrown in to stop people becoming numb-eared, or mainly songs with a few tunes tacked onto or included within a song or a couple of sets of tunes thrown in each set.