Heck, IMO even one server is risky to manage manually. If you're making all your config changes from a prompt it's easy to forget how or why you set something up a specific way and end up with a huge pain when you need to do it again. A config management tool gives you executable documentation for exactly how the server is set up should you ever need to recreate it from scratch.

>If you're making all your config changes from a prompt it's easy to forget how or why you set something up a specific way [...]

The correct way would be to just document the changes.

I tried that, but I'm not sufficiently superhuman. Things slowly but inexorably drift out of sync as changes get made in one place but not the other, despite my best efforts. Declaring how things should be and then letting the computer make it so is both less work and more reliable. (This is why I call it "executable documentation".)