If you have dozens of Safari tabs open on your iPhone or iPad and want to close them quickly, it can be tedious to press "X" on each one. Fortunately, there are two ways to close all tabs in Safari at once and it also works in Private Browsing mode. See how you can do it.
First, open "Safari" on your iPhone or iPad. While on any page, locate the tab overlay, which looks like two overlapping squares. On the iPad, you will find it in the upper right corner of the screen.

On the iPhone, it is located in the lower right corner screen.

Place your finger on the tab switch and hold it there for a moment, holding down push. At menu that appears, select “Close All Tabs”. (There will be a number in the option that lists the number of open tabs.)

When Safari prompts you to confirm the closure in another pop-up, click "Close All Tabs" again. After that, all his tabs browser you will be closed.
How to close all Safari tabs on the tab switch screen
There is another way to quickly close all Safari tabs on iPhone and iPad. First, open “Safari” and then Press the switch tab once.

On the tab switcher, locate the "Done" button, which is located in the upper right corner of the screen on the iPad and in the lower right corner of the screen on the iPhone.
Press and hold the "Done" button - place and hold your finger there for a while until a popup window.

In the pop-up window, select "Close All Tabs". The exact name of the emerging energy will vary, depending on the number of tabs that you have opened.

Safari will ask you to confirm the closure of all tabs with another pop-up message. Select "Close All Tabs" again. All Safari tabs will then close immediately. The same energy works in operation private browsing.
Source of information: howtogeek.com