The following query has 2 Custom clause keys: SUM_AGG and AVG_AGG. FNC_TblOpGetCustomKeyCount returns 2 when called from the table operator or contract function associated with this query.
SELECT *
FROM udaggregation (
on (select * from tab1)
using SUM_AGG('A', 'B')
AVG_AGG('C')
) as D;