Estimates have historically varied from 4.5% to 12%, but the best current estimate is around 10%.
People with synesthesia experience at least one sensation simultaneously with another. For example, they might hear music and see colors with particular notes, or when they see words they might associate particular tastes. Many who have synesthesia aren’t aware it is a special condition, and can go all the way to adulthood before realizing other people don’t think that way.
One of the most common types of synesthesia is “number forms”, also known as “spatial sequence synesthesia”. These are people who when hear some sort of sequence (numbers, months, years) they associate and physically see it as some exact location in space.
[Image source: Visualised Numerals, an 1881 essay by Galton.]
Keep in mind this is not some sort of artificial mental mnemonic: this is a genuine and involuntary mental effect.
The above is an approximation of the number forms of a particular person; note how the numbers positioned to give some difficulty in telling time. “Curving” initial numbers are quite common. Also note how the numbers mentally start sorting themselves by 10s; this is also quite common.
[Statistics source: Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia by Cytowic and Eagleman.]