It depends what you want to do. If you want just a witness you can run it of a decent laptop, but if you want an API Node then you should have a good machine as you will need at least 4TB of NVMe storage and 64 GB of RAM.

For basic Hive Witness all you need is this:

32 GB RAM
500 GB NVMe (SSD) storage

These are my recommended specs, but you really need only 4-8 GB RAM and 40 GB free disk space to run a witness...
Oh and ideally you are running on Ubuntu Linux and not on Windows / Docker...

To read more check out this excellent post by Marky:
@themarkymark/setup-a-hive-witness-using-the-new-rolling-block-log-feature-jte