Right. There are usually 2-3 green ones, potentially a colored one for flowering stages, a brown one for harvest ready, and a withered one. You really don't need all of the green ones. Those are just different shades of green to match the shade of green in the crop itself. For example, if you had distance2, distance3, distance4 all green, then distance5 brown, and distance6 withered. You could just rename and reference them to distance2 = green, distance3 = brown, distance4 = withered. and save 2 slots per crop done like this.
Also, some states like stubble could just use the green or brown texture depending. A lot of crops leave so little stubble you could just remove the distanceMap attribute from those altogether and have it show the terrain texture in those cases.
And yes, 5 channels = 31 crops (plus 0 or no crop), 6 channels = 63 crops (plus 0 or no crop). Also, all crops by default have 15 total growth states available (though many of them are not used by default).