Maya script. For each of the selected transform nodes, create a int type attr to the shape node, and set a running value to each.

createAssignAttrToSelectedShapes.py

import pymel.core as pm

# select the transform for all your objects before running this script

objList = pm.ls(sl=True)  # list of selected objects (transform nodes)
objShapesList = []

attrName = 'myAttrName' # this is the name of the attribute
for obj in objList:
    # loops through all selection
    try:
        objShape = obj.getShape()
        objShapesList.append(objShape)
    except:
        print ('{} has no shape nodes. skipping.'.format(obj))
        continue
    pm.addAttr(objShape, ln=attrName, at='float', writable=True, keyable=True)
    print ('attr "{}" added to {}'.format(attrName, objShape))
    
# sets a running integer value to each object shape in the list
# ie. 
# shape1.myAttrName -> 0 
# shape2.myAttrName -> 1
# ...
# shape10.myAttrName -> 9
for shapeId in range(len(objShapesList)):
    objShapesList[shapeId].attr(attrName).set(shapeId)