How to clear all the notifications once at a time on my iPhone

You can scroll the notifications until you see the X to close. Touch and Hold the X until you see Clear All. Tap on Clear All and all of the notifications will be cleared at once.

Axel F.

From the iPhone User Guide:

That one has only one method and you already know it. Those apps have to be closed by pulling UP one at a time. You can pull UP two to three apps at a time by touching them as well. There is no "all at once" button.

Axel F.