Just to add my personal experience. The church is approximately 2500 sqft with high vaulted ceilings. The organ is in the loft at the rear and everything is wood and carpet to deaden the sound
. John and I worked together on the Hauptwerk installation at my church. We installed 12 Behringer 2031A monitors and an enormous subwoofer.
On normal Sundays, with many empty pews, it is more than adequate. But on Easter and other big services with lots of bodies to soak up the sound, I feel like it gets pushed pretty hard to really fill the sanctuary with the big organ sound. I would not have wanted anything less than the 12 monitors we have.
As a side note, I have really struggled with finding sample sets that sound convincing in our space, especially at full plenum with reeds. I think it may be a combination of the sample set and too few audio channels, but many sets sound fake at full tilt in my experience. I'm guessing more natural reverberation would help with this.
This is my one and only experience with this, so take it with a grain of salt.