Mac mainframe?

No. Because it can't tell which keyboard or mouse belongs to which user.

A (much) longer "no" answer...

With a classic mainframe-style design, you can certainly telnet or ssh into the OS X Server box from thin clients or other such devices, and you can get something that's fairly close to an IBM 3270 green screen terminal display or a tn3270 emulation; one of the displays that was common on classic mainframes. (In the OS X case, it'll probably be a DEC VT terminal emulation and probably not an IBM 3270 emuation, and the VT emulation works very well for character cell operations these days. I regularly have multiple sessions open into various OS X clients and OS X Server boxes. Multiple thin clients will also work here, too.)

You could probably also use ssh port forwarding and X (or straight X over IP) and get remote X11 displays on the clients. That'd require installing and configuring XQuartz. If the remote clients are more capable (which is less common with "mainframe" solutions", then you can also use Microsoft Terminal Services-style parallel logins on Lion and later, with remote OS X systems via Screen Sharing via ARD or with decent VNC-capable clients.

Now if you want to have those gonzo-resolution displays directly driven from a central server — the core of the design used on various classic client-server computers, but with substantially upgraded graphics displays — then those designs don't work very well over the network, and for several reasons not the least of which are available bandwidth and latency acceptable to the users. Local graphics displays are very fast because they're local; they're physically short I/O buses, and more expensive. This approach will also need multiple graphics controllers installed in the host (with multiple keyboards connected) and that just isn't the expansion or the software available with any of the OS X boxes. You can end up saturating a gigabit network in fairly short order, and needing dozens of backplane slots. Resources and slots which aren't available.

These same bandwidth and latency limits hit approaches based on X11, ARD and VNC, too, though those usually don't run at Thunderbolt Display resolutions. For comparison, Thunderbolt 1 can run at 10 gigabits per second, and Thunderbolt 2 at 20 gigabits per second. Gigabit Ethermet and USB and related I/O buses are nowhere near that fast, which is why having more than a few ARD or VNC sessions becomes problematic.

If you also have a mainframe-class budget to go with the request, then many things can become more easily possible. But with a mainframe budget, simply rolling out the usual Apple solution of OS X boxes for each of the users becomes feasible, too. (Note that 3270 and VT terminals were not cheap in their day, and a typical mainframe will easily be in the megabuck range and higher, plus support costs.) I haven't seen minicomputers with multiple graphics displays, as those don't have enough of the PCIe x16 slots most graphics controllers require, assuming the mini even has any of those slots and not just x4 or x8 slots.

But if as I suspect you're looking to have a mainframe- or minicomputer-style design with graphics controllers for each user, then no Mac has enough expansion slots for that, much less software support.

The Mac GUI is not designed to be abstracted and run from a remote device (other than VNC, which would only give you a single session). Xwindows (xquartz) can do this.

You can, but not in the way you described it exactly.

You CAN have multiple users connect via VNC from thin clients running linux (rasberry pi is one example) or any other os to one central mac. I've tried it... 7 people use the same mac at the same time, it works allright, but the mac has to be fast, an ssd is a must and at least 8 gb ram - 16 gb to be safe. Your hardware is the only limit to how many can work at one time.

We also use this method to have other users watch the main user work on something and show them details of what he's working on.

Apart from the new(ish) ability of OS X to allow multiple different simultaneous VNC sessions for different users, you can also buy and install a true Terminal Server solution such as AquaConnect or iRAPP.

All three of these solutions display the session on the client computer not by using multiple screens on the server.

But, as you illustrate, it can't be done from a terminal or even a thin client. You need a fat client: one fat enough to running SSL and manage a keyboard and a mouse. Which means that the answer to the OP is still 'no'.

Simon Slavin wrote:

But, as you illustrate, it can't be done from a terminal or even a thin client. You need a fat client: one fat enough to running SSL and manage a keyboard and a mouse. Which means that the answer to the OP is still 'no'.

You can get Microsoft RDP thin client devices (e.g. Wyse) which can then connect to either AquaConnect or iRAPP.