This could depend on a few things such as what equipment you have (monitors, amplification, processing,
etc.), whether you are using existing cabling and its configuration, if it is a
monitor per mix or multiple monitors receiving the same mix, what your
mixer supports (and what is available), what processing may be involved,
etc.
Perhaps the place to start is establishing the purpose of the monitors and whether the actors and/or
orchestra are also being reinforced in the room. For example, if you want monitors covering the
stage so the actors can hear the
orchestra, in the
orchestra pit so that the musicians can hear the actors and backstage so that you can hear both the actors and
orchestra then that probably defines at least three different mixes. On the other
hand, if they were all there primarily to hear direction then they might all be on one mix.