I meant your ideal key for that particular song, because the melody can often be at the top or lower end of the chosen key. One might be comfortable with keys close to say C, but could still handle songs in G if the melody was at the high end, or even the low end but you pitched it an octave higher.
Although I know some do, you can’t really get away with learning fewer chords if you always sing in the same key, because there are various ways of playing in that key, even without the use of a capo, secondly you need to be more versatile if accompanying others, and thirdly you would missing out on a vast range of other sounds and possibilities.