At the moment, you can only use your WhatsApp account on a mobile, although nothing prevents you use the web or app version on multiple devices to check your messages, as long as your mobile is active and connected to the Internet.
For convenience, WhatsApp remembers the devices on which you have used WhatsApp Web or the PC version, so you do not have to scan the QR code every time. If you forget to log out of one of them, you can always close all open sessions on WhatsApp Web, from the WhatsApp application.
Remotely close the WhatsApp web session
To log into WhatsApp web or the application for Windows or Mac you must scan a QR code. On the same screen where this QR code is displayed there is a box indicating that the logged in session will be kept.
The good news is that the next time you want to use WhatsApp Web or the application for Mac or Windows, you should not scan the code again, but it will work directly. The bad thing is that the session will remain openSo if someone uses that PC later and enters the WhatsApp Web, they can control your WhatsApp.
WhatsApp notifies you on the mobile with a notification if the web session is active, so you can technically see if someone is spying on you through WhatsApp Web in an open session, but without a doubt the most reliable method of preventing this from happening it is log out.
You can log out directly from the WhatsApp application, without having to go to each PC where you have opened WhatsApp Web. To do this, open the WhatsApp menu and enter the WhatsApp Web section. If there are open sessions, a list will be displayed and you can press Close all sessions. If there are not, the QR code reader is displayed directly.
If you want to close a specific session and not all of them, you can tap on one of them to close it. Remember that if you want to use WhatsApp later on that PC, the process will be as easy as rescanning the QR code on duty. In summary, these are the steps to close all open WhatsApp Web sessions:
In WhatsApp, go to the chat tab
Press the menu button and enter WhatsApp Web
Press Close all sessions
Confirm in the warning window by tapping Sign off