how to have a minecraft server running 24/7 when mac is closed

You keep your Mac running 24/7, get a computer to host it 24/7, or get a Minecraft server host. This is the Apple forums, not Minecraft forums.

A Mini would be enough to run minecraft server.

If you are going to sleep your Mac by closing the lid, that may be a boit too much to run with the lid shut.

If you have an Apple Airport Base Station that is fairly recent, you can have it be a "wake-up server" and wake up your Mac for network access.

To play with others on the Internet, you will need to get your Router (whatever brand) to do port-forwarding so that the packets can get through from outside.