Looking to move some sites from shared reseller hosting to a VPS with 4 Cores, 4Gb Ram, 120GB. The traffic of the sites is 1000-3000 visitors/month, some of them Woo.

Can someone give me an idea how many sites this server can handle? 10? 50? More?

Thanks!
With that specs you can easily go with 30+ sites. As, I'm currently on managed host with half of that specs and easily managing high traffic 10 sites.
Without knowing the details of each site, this is almost impossible to tell. One poorly configured site would take that server offline with no issues, likewise if the server itself is not correctly configured. One thing to keep in mind is that 4GB of RAM and those 4 cores are also being shared by the server itself to operate, excluding actual resource usage for the sites themselves.