Edited.
I have a municipality FC and a riding FC. The spatial relationship of the data is one-to-many: one municipality intersects many ridings.
I want to do a one-to-many spatial join (a row for each intersecting riding), but I want a column in the output that indicates the Join_Count_Per_Group; I want each row to indicate how many ridings there are per municipality. All within a single geoprocessing operation. Currently, with one-to-many spatial joins, the Join_Count field is set to 1 (or zero?) for every row.