A curiosity

[tmanni]

The problem is simple : you initialize the channelNum variable inside the loop (by doing channelNum=(0)).
So, at each iteration of the loop, the channelNum variable is set to (0).
Simply move the initialization of the variable before the loop :

Code:

# integer index of channelName
channelNum = (0)

for spline in splinePoints:
        layer_copy1 = pdb.gimp_layer_new_from_drawable(layer_copy, image)
        pdb.gimp_image_insert_layer(image, layer_copy1, None, 0)
        pdb.gimp_drawable_curves_spline(layer_copy1, HISTOGRAM_VALUE, len(spline), spline)
        channelName = str(channelNum)      

        # copy Red channel as:
        channelName = pdb.gimp_channel_new_from_component(image, CHANNEL_RED, channelName)
        pdb.gimp_image_insert_channel(image, channelName, None, 0)
       
        # increment integer index
        channelNum += 1                                   
       
        # remove equalized layer that was used to create masks
        pdb.gimp_image_remove_layer(image, layer_copy1)