The first step is to determine the demand during the lead time:
Demand during lead time = (500 units/hour) x (20 minutes / 60 minutes/hour) = 167 units
Next, we need to calculate the number of containers required to satisfy the demand during the lead time:
Containers required = (Demand during lead time) / (Container size) = 167 / 35 = 4.77
We can round this up to 5 to ensure that we have enough containers to handle the demand during the lead time.
Finally, we need to add the safety factor:
Number of kanbans = (Containers required) x (1 + Safety factor) = 5 x 1.05 = 5.25
We can round this up to 6 to ensure that we have enough kanbans to handle the demand with some extra cushion. Therefore, the answer is 6.