That might be a tricky question. You see the red jewel in the toilet first. When the cutscene ends and the camera stabilizes, you can see the red jewel + 3 yellow ones. There are 3 more yellow crystals visible from the starting point (en route to the first cage), but not from the default camera angle (you have to rotate the view to see them).
So I would say there are 7 of them visible from the starting point, but only 4 at the same time?