According to specs, another 4 GB can be added, other users report significantly more RAM (+16GB).
I would like to have the following applications running on my future NAS.
-Maria DB (as an "App - i.e. via additional packages from Synology)
- Docker -> then Grafana and Influx DB
- as VM Raspimatic
+ another VM for Windows 10 Pro
Of course, it depends on what kind of virtual machines you want to run.
The thing has 4GB RAM out of the box, so the VMs would have to be pretty lean. Why do you want to run virtual machines on a NAS? As far as I know, NAS are not really designed for virtualisation.