No edit : to answer OP's question, with a simple google search for something like 'port checker' you can find a tool that will tell you if your ports are opened. If the tool says they're opened, in most cases people will be able to join. You'll still need to be hosting a server or have some service listening on the port for the port checker to work though.
From there it's just a matter of whether your internet connection and computer can handle a decent amount of players.