As you can see from the screenshot below, I have no option to select how many people can play my game at one time? When I look at the game details preview, it says it single player! its not! How do I change it ?

That's a bug! I opened an issue in our tracker so we can get that fixed: https://github.com/itchio/itch.io/issues/557

Great Amos thanks for letting me know! I though i was going crazy :)