I would use the GetAllActorsOfClass in the BeginPlay to store a integer TotalColectibles contaning the quantitny of all the collectibles in the level (considering this value won´t change) and then in another integer CurrentCollectibles you add +1 everytime you get a new collectibles and in the same function check if TotalCollectibles = CurrecntCollectibles. This way you will be able to use the variables to show to the player in a HUD