The 1st day if a monk did not see a monk cursed during the prayer in common he would deduct that he is (cursed) thus he would commit suicide.
The 2nd day if another cursed monk sees 1st still living he will think that he has seen an other one and that can be only him.
Thus they would commit suicide both the 2nd day.
Similarly: the 3rd cursed monk, if he saw 2 cursed monks the previous days and if they are still alive, will know that he is cursed.
Thus in fact n monks commit suicide the nth day.
As they commit suicide on Sundays which is the 7th day, there are 7 cursed monks who commit suicide.