rayquaza..veronica ==
50
Fifty floors including the summit.
there are 50 floors, afterwards you will fight rayquazza. if your starter is you team leader, he will automatically ask to join your team.
there are 24 floors in temporal tower.
100
Is it from Pokemon mystery dungeon or from real life
In Pokemon Mystery Dungeon "Something is approaching" Does not really mean much at all, except "GET TO THE STAIRS NOW!" because if you don't, then you will be defeated from nowhere.
You have to use the stairs to reach the last floor.
At Sky Stairs Summit through a nintendo event
It means that an unknown force is will push you out of dungeon if you don't reach the stairs on time.
Sky Stairs Summit
there the "x-ray specs" and they reveal hidden stairs and items