There is no 'best'. It really depends on the person. Your 40 might be another's 80. Besides, the number alone does not indicate how long do you spend per repetition. A quick breath can be worth a quarter of a long breath, and someone can't match someone else's quick breath with their long breath.
When we look at numerology, the number nine represents endings, so multiples of nine are good options. However, there are many more power numbers, and not all of them are multiples of nines. At the end of the day, whatever amount you choose, as long as you do it as well as you can for as many days as you need to it will bring as much as you can get out of it. Certain numbers are a good final touch, but not decisive to the working at hand.