There are two secret rooms in each level of the game, and they always have at least one shopkeeper. Secret rooms are identifiable by a crack in the wall that can be blown up, but not every crack is an entrance to a secret room.
Mobile Gaming eGuide - Tips, Tricks, and Strategies for Today's Hottest Mobile Games The Binding of Isaac: Rebirth
There are two secret rooms in each level of the game, and they always have at least one shopkeeper. Secret rooms are identifiable by a crack in the wall that can be blown up, but not every crack is an entrance to a secret room.