What hardware minimun requirements do you recomend for one instance of geoserver and postgis and 20 concurrent users? Vectorial data stored in PostGIS is around 20 MB only. I need the ammount of RAM, disk space, bandwith, server cores and processor velocity.
That depends on a lot of things. But if all your data is 20 mb you shouldn't need very much of anything.
Maybe java and geoserver will need some ram to feel good. But PostGIS shouldn't need any more than the OS needs for such a job.
So if you run some Linux distro I don't think you will find a computer that can't handle that load.
Maybe 1 GB of RAM is recommended and any decent 5 year old cpu. For disc you will need maybe 7 GB to have some extra space for future updates.
How fast your users will experience your maps depends more on how you build things than the hardware.
Will you serve in some vector format or as wms-service or what. Will you cache the output? and so on....