I'm going to say 4...

A Macintosh Plus for systems 1.0 to 7.5.5

A LC475 will support 7.1-8.1

a Blue & White Powermac will take 8.5 - 10.4.11

A 2008 Mac Pro will do systems 10.5-10.10'

These systems will support these Mac OS versions without any "Hotrodding." Of course they might need a ram upgrade...