Harvard dictionary of music give vocal ranges as follows:

Soprano C - A
Mezzo soprano A - F
Alto F - D
Tenor B - G
Baritone G - E
Bass E - C

This is a range of thirteen notes for each vocal compass. Some singers have more than this and some less.

If the song has a range of notes exceeding thirteen, most non trained singers will have difficulty in any key. If the range of notes in the song is an octave, there are five notes "headroom" and a singer with a range of thirteen notes could sing it in about eight or nine keys. If the singers range is an octave, they will only be able to sing the song in one key.

There is substantial overlap of range between the following so a common key would be easy to find:

Soprano - Tenor
Mezzo - Baritone
Bass - Alto

This assumes songs sung using the same tune ie without harmony.

A singer does not have the same key for all songs. The key is changed for vocalists so that the top note of the melody is =< the singer's top note, and the bottom note of the melody => the singer's bottom note