I believe it's stated somewhere what's recommended for the server. If you're hosting it on your own PC I'd recommend getting more RAM yourself. e.g. 12/16GB and split it 50/50 for server & client.
For a dedicated server/VPS/any other server somewhere between the range of 4 to 8 should be fine. Although I wouldn't ever use a dedicated minecraft host. Since those prices are through the roof. A simple VPS is way more worth for your money. But inclined to say you'd need to have some knowledge regarding Unix. LinuxGSM does help out a lot and provides with a good guide. Minor adjustments needed from their default Minecraft JAVA server. Which I'm currently using.