DBPro might be able to handle 50 players, or it might be able to handle 500. It depends on the complexity of the game and of course how you write the server application.
You would basically use DBPro and DarkNet to make a server application that takes in data from every player, and sends it out to all the other players. It sounds simple, but when you think about it, that's a lot of data when you get more than just a few on.
To improve efficiency on the server you'll want to separate the game loop from the display loop. Only display the necessary text on the server display to let you know whats going on. This code snippet I wrote for
Decoupling the Display Loop from the Game Loop is a great method for doing that. A display rate of 5 FPS for a server is plenty.
You'll want to install Windows XP or Windows 7 on the server computer. DBPro apps don't run on Windows Server. You can also install MySQL on the same machine.
In the server app using DarkNet, you would specify a port for the clients to connect to. When you're ready to have people on the internet connect to it, you'll want to forward that port on your router to the computer that's acting as your server.
Then write the client to connect to that port on your ip address.
I can't really go into to much more detail than that. If I need to then you may not have the skills necessary to pull it off at the moment. Which is ok, you'll just have to understand that there is a lot to learn to get where you want to be, and it's going to take time.