What are the constraints that limit the number of cones in the human eye AND what strategy is employed to maximize color and fine detail vision?

The cone cells are photoreceptors that are not sensitive to light. They are sensitive to three different colors, which includes red, green, and blue. The signals from these photoreceptors are transmitted to the brain, which are then perceived as colors. The cone cells function only in bright light. The fovea is a regions where only the cones are present and rods are absent. This region is responsible for providing clear vision.

Three types of cones are collectively responsible for discriminating between different colors

S-cones: preferentially sensitive to short wavelengths (e.g., "blue")

M-cones: preferentially sensitive to middle wavelengths (e.g., "green")

L-cones: preferentially sensitive to long wavelengths (e.g., "red")

The rate of absorption of photons is determined by various properties of the cone cells as follows:

The size of the cone aperture has two effects on processing of the retinal image. First, large apertures cause an attenuation of high-spatial-frequency information. Second, large apertures allow individual cones to collect more light.

The spacing between cones determines the resolution with which the retinal image can be sampled.

Both cone aperture and spacing determine the retinal coverage, which is the proportion of retinal area covered by cone apertures.

Humans experience monochromatic vision at night, under scotopic or dark adapted vision when only a single type of photoreceptor (the rods) is active. As only one receptor is involved, the key constraint has to do with the receptor sensitivity peak and breadth within the span of solar radiation.

A second possible constraint is the range of chemical variation in photopigments.

A third constraint has to do with the span of visual pigment sensitivity, because the sensitivity curves must overlap to create the "triangulation" of color.

The fourth constraint is avoiding useless or harmful radiation. 

These are overcome by trichromatic vision, which creates a unique combination of cone responses for each spectral wavelength and unambiguous hue perception. This enhances object recognition when surfaces are similar in lightness or are randomly shadowed, as under foliage. It also substantially improves the ability to separate the color of light from the color of surfaces, because illuminant metamerism is also reduced, color constancy is greatly improved.

Another important trichromatic benefit is that it reduces metameric colors to various flavors of gray around the white point and into dull blues and purples.