Humans are usually trichromats; they have three different color receptors in their retina. However, there are the exceptional few who are tetrachromats.
Tetrachromats can see up to 100 million colors, which is 100 times that of the average person.
Tetrachromatic eyes are usually present in several species of insects, fish birds, amphibians, and reptiles.
It used to be the normal for most mammals to have those eyes long ago, but it became rare due to genetic mutations.
Also, since the cone cell pigment genes are only present on the X-chromosome, tetrachromats are usually females.
Since researchers are still trying to understand tetrachromacy, there aren’t any hard numbers on how often it manifests itself.
If you’re curious about whether or not you’re a tetrachromat, I suggest participating in a research study instead of taking an online tetrachromacy test since, according to researchers at New Castle University’s Tetrachromacy Project, “computer screens do not provide enough color information to be able to ‘tap into’ the extra dimension that tetrachromats may possess”.