Изглежда, че искате общата сума. Можете да използвате аналитичния SUM
функция за това.
select mnth,
sum(stmts) over (order by mnth) stmts
from (select TRUNC(ACTHX.STMT_HX_STMT_DATE, 'MM') AS MNTH,
COUNT(ACTHX.INVOICE) as STMTS
from ACTHX
group by TRUNC(ACTHX.STMT_HX_STMT_DATE, 'MM'))
ORDER BY mnth