you can announce 6 drivers. 4 of them are allowed to drive in the 16 hours this year. You can choose free which one drives. This is mainly to have less 'last minute changes' because some drivers get to know that they can't drive only that day. We had the problem last year, that a team started and didn't finish, because they didn't have an announced driver that had time for some of the last hours. If they had asked me, they could have ended the race with another driver and i would have changed that, but if many teams do something like that, its impossible to keep track of everything.
So you can announce more drivers than you are allowed to use, just to be sure you have got enough even if 2 drivers can't start.