南 無 阿 彌 陀 佛
The most important thing in chanting Namo Amituofo is to focus.
One should be fully concentrated and have no other thoughts during chanting.
It is normal to have thoughts coming up during chanting, so one should use Namo Amituofo this name to focus.
Remember, Qantity is not as important as quality.
Please chant at the multiple of 7, such as 7 times, 21 times, 49 times, 108 times or 324 times.
A Buddhism prayer bead usually has 108 beads, which is one cycle. Hence, 324 times equal to 3 cycles of a prayer bead.
An other method is to chant by minutes, such as 5 minutes of Namo Amituofo.