"CollectableSprite" is your sprite, I don't know how you named it in your project (could be "Coin" or something else).
".Count" is an expression, which return the number of sprite instances on your layout.
Create a variable TotalCollectables.
Add this event to your "On start of layout":
Set TotalCollectables to CollectableSprite.count
Then you can display TotalCollectables in that text:
System-> every tick -> define text : Player.PlayerCoin & "/" & TotalCollectables