That depends on you.
You can get a maximum of 100 discount per month. So, if you make a utility bill payment of 500 or more on a given Friday, you have reached your limit for the entire month.
But, if you make a payment of less than 500, let's say 300, then 20% of 300 is 60. You can still get an additional 40 discount on your next transaction made on a Friday.
In short, there is no limit to the number of transactions. You can use it as many times as you wish, but once you reach 100 in discount, you cannot gain any more in that particular month.