You can use remote control solutions like VNC (free and open source), RDP (proprietary), etc., but these are all software-based solutions that don't always support every platform and won't provide you with access to change pre-boot settings in the BIOS.
I find that the best way to control a number of machines at the same location effectively is to use a hardware solution known as a KVM Switch, which also supports pre-boot and post-boot environments (all Operating Systems and video modes) without the need for any software or drivers.
- K = Keyboard
- V = Video
- M = Mouse
Pictured here is a 4-port KVM Switch by a company called ATEN. It's an older model (I have the 8-port version of this series, and it has been very reliable for me) that uses PS/2 mouse and keyboard ports. They also have newer models that support USB mice and keyboards, and different video technologies as well:
If you have enough space, you can simply not connect the Video cable to the KVM Switch and just keep each computer directly connected to its own video display(s) instead (which will help to further increase your effective usage), so you do have choices. Having just one keyboard and mouse to control all the machines is a nice convenience though.
Some of these KVM Switch products have IP support and will work over the internet, but the price starts to climb upward very sharply for these types of units.