one per computer, but several computers. from old legacy systems (some of which predate the 8086) to high end win 7 gaming machines and pretty much everything in between.  386 and slower machines are pure dos or some earlier OS that predated dos (BASIC and the like for pre 8086 stuff),
80486DX to K5/pentium 100 - it depends.  slower than DX2-66 and its pure DOS, DX2-66 and higher, its Win ME or some lightweight linux distro.

Pentium 100 - PIII (Katmai) always win ME

       PIII (coppermine) P4 wilamette/early Athlon Linux distros.  I had bad luck with win9x on coppermine/celeron and early athlon

     PIII (Tualatin)/P4 (Northwood/prescott)/ AthlonXP/early Athlon64 -Win ME (sometimes with Debian linux dual boot)

S939 Athlon64/S775 Intel to AM3+  Almost always Win 7, unless the specific chipset happens to hae drivers for ME (There are a handful of them that work surprisingly enough)
If its a supported system then dual-boot win ME and win 7 otherwise dual boot win 7 and KDE linux.

There are some things you just can't get away from native win9x for, and virtualixation/hypervisors just won't work for. (Though the LLE project 86box is certainly trying to get natie win9x running as it should in an emulated eniornment. albeit slowly (best my Fx9350 will do is a mobile pentium MMX @166 and voodoo 3 graphics) so currently native win9x is still the best option.

This topic has a misleading title, and if you don't read the first post you will miss that it is involving virtual desktops and not physical computers.

I have 2 PCs that have VirtualBox and I'm going to presume running DOS does not count.

Also not certain if RDP counts as 1 PC can RDP into 2 (physical), while another one can RDP into 2 VMs that are running on Hyper-V off another system. I don't use these as actual "Desktops" tho, it is only if needed as all of these are Servers and anything I need to do can be done via network shares.