Currently I'm using Karma Render Settings node and usdrender rop node for my renders.

If I set to use cpu there is no option anywhere to be seen that I could find that has the same parameter as with Mantras'
There is...but's greyed out being unchecked by default; Under the Driver Tab.

'Use Max Processors', so that I could keep at least one processor open for other tasks while rendering.

I'm guessing that it's a low priority option that hasn't made it yet to be included,

or maybe the nature of USD rendering workflow makes it difficult to implement?

If rendering via husk is an option for you, the -j parameter does exactly that (use -j -1 to use all available threads except for one).

Ok cool.
I've not used command line rendering.
So if I go the route of husk rending is that basically just replacing the usd render rop?
Do I have to make a script that outputs 'flagged text' to get the same settings as put on my usd render rop/Karma Render Settings ROP.

Maybe there's a short tut video the demonstrates the relationship between what I have in a LOP network and my console commands for husk rendering, or I guess just looking through the docs should be good enough.

On the USD Render ROP, under the "Driver" tab, enable "Maximum Threads" and set the max threads you want to allocate your render process.

You can always modify the render command too to add any missing features that are added to husk but that haven't been exposed on the USD Render ROP yet. So where it says "husk", you can replace it with "husk -j 4" for example - however if you do that, make sure not to enable "Maximum Threads" or it'll be specified to husk twice and I can't say which one will win out)

To find out all of the options available (most of which are exposed on the USD Render ROP mind you), in a houdini command shell, type