How to Control Multiple Computers with a Single Keyboard and Mouse or computer?

For that you literally have to use one computer , and from that computer you may login to others remotely and control them . That's a way to control multiple computer from a single computer & of-course with a single keyboard and mouse .

You can get a KVM box.  This box connects more than one computer to one keyboard.  For example a 4-in-1 KVM connect 4 PCs using one 1 console:  one mouse, one keyboard, and one monitor.  There are many different varieties, such as more PCs, or connecting using usb ports instead of the legacy PS2 round ports.

If you don't have the KVM hardware and still want to connect to PCs remotely, you can use remote desktop (only available on Windows Professional).  If you have Windows home, you can remote using  software like TeamViewer 

you can do that using ssh protocol or using some remote applications like remmina(linux), Remote Desktop (windows )

You control it but you cannot control it at same time You can control them one by one at one screen by taking every computer on remote