I’m a great believer that almost anybody can sing: it’s more of a craft than an art and the more you do it, the better you get. I stumbled across this 2011 piece, which suggests I’m wrong about 5% of people.
NBC News: Why some of us are terrible singers
[A] study found that anywhere from 40 to 62 percent of non-musicians were poor singers, a rate much higher than shown in previous research.
It also found that roughly 20 percent of people can’t sing accurately because they don’t have good control of their vocal muscles. Another 35 percent of poor singers have trouble matching the pitch of their own voice to the same sound heard in other timbres, such as when it’s coming from a trumpet, piano, or a person of the opposite sex. And 5 percent of lousy singers lack the ability to hear differences in pitch or discriminate between two different sounds.