Display the top two most popular activities across all reservations

The Table is

SUPERVISION (ResNo, ActivityID, SupervisorID, Day, Time)

I have done something like this but it is wrong

SELECT COUNT(S.Res, S.ActivityID) AS PopularActivities FROM Supervision S WHERE rownum = 2; ORDER BY COUNT(*) DESC;

or

SELECT S.ResNo, S.ActivityID FROM Supervision S WHERE (rank() over (order by count(*) DESC) as RNK from Supervision S) AND k = 2;