question

Have a look at your "Max instances per host" and "Min free slots" settings. "Max instances per host" is how many instances of your executable can run at once on a server host, while "Min free slots" is how many non-running slots need to be available at any time for incoming players (so that when someone calls Matchmake, they don't have to wait on a server host to be requested, provisioned, and started).

So, if "Min free slots" is close to (or over) the "Max instances per host", you'll likely have multiple servers running while testing. You can explicitly set the maximum number of servers to 1 during testing if you like - that's in the Settings->Limits tab (look for "Game Server Count").