You can use the following to calculate the rework for a certain activity

CALC_REWORK( ACTIVITY_TABLE.ACTIVITY = 'Activity A')

Taking SUM for the same would give you number of rework activities for all cases

SUM(CALC_REWORK( ACTIVITY_TABLE.ACTIVITY = 'Activity A'))

I wonder why they call it "CALC_REWORK" when it returns the total number of activities per case.

Of course if you filter per a specific activity, then you will get the rework, but... still

I use more often INDEX_ACTIVITY_TYPE when returns more than 1

because, if an activity occurs more than 1 time in a case this is rework.