I need more than one game instance in my game one is for saving and loading the game and the other one for connecting to the database it has only the user-id, but I should only assign one game instance to "game instance class " in the project settings, so how can I use both of them?
You can’t!
There’s only one in project settings for a reason. There can only be one game instance.
It is what it says it is - an instance of the game. If you have a multi-player game, there would be one instance of the game for every client and probably at least one dedicated server.
Sounds like you want to make a session
Then you can find a way to do what you want using what’s available. For example, the game mode is a decent place to store ID’s and such information.
If you need more clarity, check out the docs and reply here and I’ll do my best to clarify.
Otherwise, please accept my answer if it has helped you out!