There isn’t a minimum amount of Miles required. So long as you earn or redeem your Miles at least once every 18 months*, your Miles will remain active.
(Remarks: *For all new Miles earned from 1 January 2020, the Miles expiry date is automatically extended by 18 months from your last Miles crediting or debiting date. All Miles credited on or before 31 December 2019 will continue to follow the former Miles expiry arrangement.)