I would suggest the answer is two..

Either you have mirrored two spindles two times and then striped the resulting array (RAID 1+0) is which case everything that happens to one spindle MUST happen to the other synchronously or you have striped two spindles two times and then mirrored the results (RAID 0+1), in which case a single write will also have to be written to two physical spindles.

BTW - doing the mirror 1st and then the stripe actually increases your reliability once you get to 6 spindles.