By default, the first four actors are the battleparty - everything above four is the reserve and doesn't fight unlesa placed in one of the first four slots by formation screen.
To change that, you'll need a plugin - for Ace there was a party manager script with that option, but I don't know if something like that has been written for MV yet.
I've moved this thread to Plugin Request. Please be sure to post your threads in the correct forum next time. Thank you.