Ahh.. right, single chainring. Regular cassette or freewheel and derailleur on rear.

So... 1x3, 1x5, 1x6, 1x7, 1x8, 1x9, 1x10, or 1x11.

No doubt one could get quite a range if one goes with an 11 speed with 11-28 or 11-32 on the rear. Then choose the chainring that would fit the rider's preference.

Get a big enough cassette sprocket on the rear, and there is no need for the second chainring, but one probably doesn't want to go super-small either.

As mentioned, Capreo goes down to 9T. There are a couple of companies that have also put 10T on their cassettes. SRAM may be one of the big vendors with a 10T cassette and a mountain gear range. But, you're probably better off going with the more standard 11T if you can get the right chainrings.

I find that 144BCD is a bit of a pain to get parts for. 130 is much more widely supported.