I had a similar problem after an update where 3 of my 18 cameras went down, I tracked it down to H.265 not working anymore with BI, and I had to go into the cameras and switch to H.264 and they started working again. They went down like 3 updates ago now. on version 4.6.9.5 currently.
I currently have 16 2mp cams in my BI system, and adding an additional 7 4MP cams this week. But I may have to add more cams in the next 2 months.
What kind of hardware are people running for 30+ systems? Currently have a I7-7700