How can I provide many Windows desktops for the cheapest possible price?

I could provide a detailed HowTo for the best solution I know of, with screenshots and step-by-step instructions, but this would require an enormous amount of effort, and ThinLinX would derive little benefit.  We are not a free thin client consultancy!

I could write a very lengthy essay on the Thin Client concept, common misunderstandings and poor implementations.  I'm sure that you can find plenty of opinions about this on the Internet.  The really, really short version is that thin client isn't a magic cost-saving trick - it's about altering your cost structure to spend more on the server room and less on end-user workstations (by which I mean the stations at which people work, not just the PC that they use), with notable implications for information security.

There is no magic way to run many Windows desktops at no cost - if it looks like that, it's only because you're ignoring an historical sunk cost for acquiring Windows desktop licenses (e.g. Windows 7).  Contrary to common poorly informed public opinion, Windows 10/11 isn't "free", and if you use multi-session Windows Server (with the RDSH role installed) instead, then it's going to cost you almost as much as the same number of separate desktop licenses in RDS Client Access Licences (RDS CALs).  Microsoft has that loophole covered.

Broadly speaking, you need to use some form of free virtualization (I'd suggest VMware ESXi, but there are some alternatives) to run either (a) multiple instances of a desktop O/S such as Windows 10 (possibly upgraded from Windows 7/8/8.1), or - my preferred option - a single instance of an older version of Windows Server with the RDSH role installed, plus a virtual domain controller, with cheap but legitimate RDS CALs (FYI I bought a 50-user Windows 2016 RDS CAL pack on eBay for only USD ~$60).

By way of indication of what is possible, I have a single NUC8i3BEK (approx 120x115x35mm, not including external power brick) with a 1 TB NVMe SSD and 32 GB of RAM that hosts my entire Citrix HDX and VMware Horizon test suite, under VMware ESXi 6.7.0.  I only need to run three VMs (a domain controller, a Horizon Connection Server and an RDS that serves as a combined Citrix DDC/Citrix VDA/Horizon host) to test pretty much everything.

Note that VMware have no interest whatsoever in supporting ESXi on anything other than "server-grade" rack-mount hardware, and if you choose to use VMware ESXi on anything else, you will almost certainly have to install unofficial community-supported VIBs for SATA storage and/or Ethernet.  You can create a custom ESXi DVD that contains such drivers, but the procedure for doing so is non-trivial.

Performance and scalability is not great, of course (considering the hardware), but if I dispensed with the unnecessary expensive COTS software (for which ThinLinx has free Not-For-Retail licenses by virtue of being a partner) I think that I could easily host 10 concurrent RDP sessions with acceptable performance even on this grossly underpowered hardware (my focus is portability, not scalability!).