Hi,
let me add some more information
Can we use other technologies or ways to run robots?
It is not clear to me what you are looking for. Other ways to run robots - from windows scheduler, through HTTP.
There is also the VMware virtual desktop that exist.
You can run it within VMWare too.
Can we run also robots on Linux?
Ha, ha, yes, as long as you run virtual box (or any virtualization technology) with Windows OS within Linux ;),
Can we run multiple robots on 1 machine?
Well, actually you can but it requires more thinking and deeper knowledge of combination background, foreground, exclusive options when running Business Objects and Processes. I always ask what would be the benefit (I can hear you - lower number of machines to take care of) because you would need carefully consider which processes can run in parallel on one machine and do not conflict between each other - stealing focus, keyboard, etc.