virtualbox.org

I have 20 machines running and im interested if there is a way to install guest additions for all of the machines in 1 go.
It will take quite a long time to do it 1 by 1.
Thanks.

Guest Additions (GAs) are like any other program that needs to be installed on a (virtual) computer. So, if there is an Unattended Installation option, you should explore that. Luckily, there is one for VirtualBox GAs. Check Ch. 4.2.1.3 Unattended Installation in the User Manual.