When you place an entry bracket order, there is only one order number, so irrespective of how many trades it takes to fill, the brokerage charged will be only Rs 20.
But for every fill there is a separate exit/SL orders placed, with different order numbers. So every exit/SL when executed will be treated as a separate order for calculating brokerage.
So if an order to execute got filled in 5 trades, entry order only Rs 20 brokerage, since there are now 5 separate exit orders, brokerage will be 5 x 20 = Rs 100.
This response I got from zerodha team as part of support ticket.