20 minutes isn't sufficient, need at least 40. And you need a spare set, so you need 7.
Cork needs 7 + 1 spare which is the MK4 fleet, its currently running as 5 or 6 plus some ICR's
9 car is used for GAA specials regularly. Dublin Cork line only
8 car sets are used on Saturdays during the engineering works between Heuston and Portlaoise, again somewhat restricted in routes they can operate