Today a bug started happening where when starting the Local Server test the player count option would be ignored. Instead it would always create only one player, which is also the option which appears to be selected by default in the test windows (but not in the main edit-mode window where I chose something different).
Also, I think sometimes attempting to add new by players via the test windows (by clicking the start option with the default one player option selected) would create more than one player (I think it created the number of players which were originally chosen when the local server was originally being created from the main edit-mode window).
I think this bug happens per studio session. So you can reset it by restarting. When it happens, it will keep happening consistently for a while, but randomly might go back to working again later. That just happened to me (the bug randomly fixing itself and studio correctly trying to create the right number of windows / players), except I started getting stuck on the login screen in the new windows and so was unable to run local servers at all (Edit: Studio had just logged me out entirely for some reason, so I had to login again upon restarting).