There's never to much cameras!
Currently I have 10 cameras running.
I have a few (more than a few) that I haven't installed yet, but when I finally will install them I'll be running 18.
And.. That's my next 'problem' as my NVR only has 16 channels.
So, i'm looking at either buy a 32 ch NVR or to start experimenting with
Blue Iris.
In my case I mainly use the cameras to see what's going on on my little farm, to see that the animals has water/food and to make sure they're where they should
My wife loves the cameras as when she's in the bed at night and hear noise from the stable she no longer need to either worry about it/ or o out and see what's going on, just pick up her phone and check the stable cams.
Edit/Addition: I'm a Dahua guy and so far my favorite cameras are the SD59432XA-HNR that I've been running since a year, a few weeks back i also picked up the 7442H-Z4 - to describe it in short its just Insane! =)