If you shop for acrylic markers you will find sets that include many colors; a set I saw most recently has 156 markers. I have chosen to limit the number of colors in my marker sets. I have found that most 12 ct. sets include the same 12 hues and that is enough for me.
So why do I not choose a set with a wider range of colors? I have three personal reasons.
INTENSITY: To achieve a bright, clear and consistent color, I can use any of these 12 markers opaquely. If I wish to shift an area toward a more muted tone, I use my black, gray or white markers with one of those 12. I can blend or hatch or do a wash with these neutral tones. I achieve a wide range of color with markers without all the color mixing that I wasn't very good at with paints. If I wish to make the color more intense in an area, I simply layer a pure hue hatched color over the muted area. In this way I can go back and forth, blending or hatching, until I achieve the tone I wish. The variations are near endless.
CONSISTENCY: Keeping a consistent palette within a piece is difficult for me with too many color selections. When I took a painting class, one of the first assignments was to use only two colors to complete a work. That assignment was purposeful. It proves that it doesn't take many colors to achieve a wide range of color and value, and with fewer colors the piece retains a unified and consistent appearance. I've found the more color mixing I did with paints, the more likely I was to lose the unified appearance of the piece. The work didn't convey that it had a single light source. Also, I was frustrated having to remix a color, especially with acrylics, because they dry darker. With a limited number of markers, I don't end up with the same frustrations I fought against with paints.
GAINING MASTERY: The fewer colors and tools I use, the more mastery I gain over them. I know what the red marker looks like, because I don't have 15 shades of red markers. I have one or two.
What if I find myself longing for a color I can't seem to achieve? As I mentioned in other posts, I use colored pencil to shade and blend with the markers as well. Shading with the colored pencils never prevents me from going back to my basic set of markers to "reunify" the work as I'd like. And often I'll find a subtle blend that's just right.
To recap, fewer colors mean creating a more unified piece, easier modifications to a work, and quicker mastery of the tools.