Each eye contains three receptors (one for each primary color) that generate the experience of color when stimulated in various combinations. This is known as the Young-Helmholtz Trichromatic Theory. Those who have defective cones have difficulty seeing certain colors and are known to be color-deficient. With this in mind, it is fair to then say that the number of colors the human eye can discriminate depends mainly on the sensitivity of the individual's eyes. Visible light is an electromagnetic wave that has a wavelength range of approximately 380 nanometers to 740 nanometers (refer to the table below).
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The tremendous variability in the spectral composition of light reflected from surfaces lends itself to eliciting a daunting gamut of more than 100,000 discriminable colors, and the variation in the names we assign these colors is limited only by scope of human experience.. 100,000. Wyszecki, Gunter.
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Subtractive (Pigment) Color Primaries. However, there is another set of primary colors with which you may be more familiar. The primary colors of pigment (also known as subtractive primaries) are used when producing colors from reflected light; for example, when mixing paint or using a color printer.
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Transparent yellow is ASTM number PY150 made from the nickel  element. Transparent magenta is PR122, C20H … 12N2O2. Transparent  yellow and transparent magenta mix into the secondary pigment color  red. The 3rd transparent primary pigment color is cyan, made from  the copper element, PB15.
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There are seven colors defining wavelengths of visible light. They are red, orange, yellow, green, blue, indigo, and violet. However, the three primary colors for ADDITIVE combination (such as the colors on these web pages) are red, blue and green. All edited colors in computer images use a combination of these three.
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Yellow, magenta and cyan are the primary pigment colors, red,  green and blue are the secondary pigment colors. Light uses the  opposite colors, the secondary pigment colors become the primary  light colors. Red, yellow and blue are not the primary colors in  pigment or in light.
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The primary colors of pigment are magenta, yellow, and cyan (commonly simplified as red, yellow, and blue). Pigments are chemicals that absorb selective wavelengths—they prevent certain wavelengths of light from being transmitted or reflected.
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There are three primary colors-red, blue, and green-that make the millions of colors that are distinguishable by the normal human eye.
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Red, green, and blue are the primary colors of light—they can be combined in different proportions to make all other colors. For example, red light and green light added together are seen as yellow light.
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Different wavelengths of light are perceived as different colors. For example, light with a wavelength of about 400 nm is seen as violet, and light with a wavelength of about 700 nm is seen as red. However, it is not typical to see light of a single wavelength.
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number of primary colors
There are three primary colors.  The number of primary colors is three.