Apple usually doesn't say they are sold out, you can just infer it from the availibility dates on the website (e.g. if the store says shipping in three to four weeks). If a product is successful (and all their big products everyone cares about have been, so they have always been reporting numbers on iPhones and iPads) they will usually report a nice round number after the opening weekend (like "We sold three million in three days!").

They will say when they are supply constrained, but that's during their quarterly calls when being supply constrained is properly interpreted as a more negative than positive thing.