We just upgraded our media computer to a 2018 Mac Mini with an i7 processor at 3.6 Ghz, 16 GB ram. I am running 2 HD displays and a projector as the 3rd display. Normal service I run prelude music on iTunes, media slides (and occasional video) on ProPresenter and Audio Hijack to record the service. From past experience, we may at times run Skype to PP and sometimes have to access email to get slides.
I use a second Mac to run a lighting controller-pretty simple lighting just platform floods in 2 groups and house lights. This Mac is also slated to run a PTZ camera or 3 to give us basic video feeds. I might run prelude on this one instead-just to give me flexibility in how I set up to 2 audio feeds.
I think workflow would be better if I had the lighting on the mini-we typically use a midi board to trigger that-just nervous about everything playing nice. How much is too much in one place?
You'll know when it starts crashing.
For me it comes down to usability. Your system may be able to adequately handle everything running on it but can it be effectively operated/managed/etc that way? Sometimes single purpose systems are nice because it's much easier to manage interfacing with the system.
Running multiple audio sources from one computer can be obnoxious as you have to kill one before you start another. Adding a multi-channel audio interface (or going direct to the console with USB if possible) can help alleviate this by binding each program to it's own audio channels. Makes it easier to mix too as different source programs can result in vastly different levels. Of course you need enough channels on your console to pull this off as well.
Another way to think about things - if you really need X number of operators (lighting, gfx, video) then you really need 3 systems. If two people can do it effectively then you may be able to get away with two systems. Still may need 3 monitors though.
A lot of it will come down to how complicated your services are. I personally like prelude to run from a simple computer sitting at FOH and run by the audio person. Gfx doesn't need any part of that business. Audio can pretty effectively run smaart, spotify, wwb, wsm, dante dm, and some other bits and pieces pretty easily with two displays. I've got that all running (except smaart) on an old Win7 box with a single display and it's quite happy. Seriously thinking about adding a second display and running smaart on it too, not at all concerned about stability there. It is however, not a mission critical system. If it does go down service will go on as the only thing the congregation would notice is a blip in pre-service music which is cut 5min before service for a pre-service video to run anyways.
That brings me to my final thought. Back in my IT days, critical systems all ran on dedicated hardware. Less critical stuff got to share.